Fix serialization allocation in .NET Framework and < .NET Core 3.1 (#7884)

## Summary of changes

- Fixes "incorrect" generated code from `TagsList`
  - Removes significant additional overhead during serialization
- "Fix" vendored System.Buffers code to avoid the same issue

## Reason for change

The current generated code for `TagsList` produces something like this:

```csharp
private static ReadOnlySpan<byte> DbTypeBytes => new byte[] { 167, 100, 98, 46, 116, 121, 112, 101 };
```

This _looks_ like it's allocating a new `byte[]` with every invocation,
but the compiler actually optimizes this away to be completely
zero-allocation, by embedding the array as part of the dll, and then
simply returning a `ReadOnlySpan` wrapper pointing to this fixed data.
You can see this if you look at the generated IL:

```
  .method private hidebysig static specialname valuetype [System.Runtime]System.ReadOnlySpan`1<unsigned int8>
    get_DbTypeBytes() cil managed
  {
    .maxstack 8

    // [20 58 - 20 109]
    IL_0000: ldsflda      int64 '<PrivateImplementationDetails>'::A06A154BE3B860D0B56FA96C93523B732045BA0BCE2FFD4769109575CF1953BF
    IL_0005: ldc.i4.8
    IL_0006: newobj       instance void valuetype [System.Runtime]System.ReadOnlySpan`1<unsigned int8>::.ctor(void*, int32)
    IL_000b: ret

  } // end of method SqlTags::get_DbTypeBytes
```

However, in .NET Framework, even though we have vendored
`ReadOnlySpan<T>` so we can get some of the benefits (mostly cleaner
code), we _don't_ get these benefits. Which means that the above code
_does_ generate a new array with every invocation:

```
  .method private hidebysig static specialname valuetype Datadog.Trace.VendoredMicrosoftCode.System.ReadOnlySpan`1<unsigned int8>
    get_DbTypeBytes() cil managed
  {
    .maxstack 8

    // [20 58 - 20 109]
    IL_0000: ldc.i4.8
    IL_0001: newarr       [netstandard]System.Byte
    IL_0006: dup
    IL_0007: ldtoken      field int64 '<PrivateImplementationDetails>'::A06A154BE3B860D0B56FA96C93523B732045BA0BCE2FFD4769109575CF1953BF
    IL_000c: call         void [netstandard]System.Runtime.CompilerServices.RuntimeHelpers::InitializeArray(class [netstandard]System.Array, valuetype [netstandard]System.RuntimeFieldHandle)
    IL_0011: call         valuetype Datadog.Trace.VendoredMicrosoftCode.System.ReadOnlySpan`1<!0/*unsigned int8*/> valuetype Datadog.Trace.VendoredMicrosoftCode.System.ReadOnlySpan`1<unsigned int8>::op_Implicit(!0/*unsigned int8*/[])
    IL_0016: ret

  } // end of method SqlTags::get_DbTypeBytes
```

This is... Bad 😅 And it explains the _significant_ serialization
overhead identified in #7882 for .NET Framework. I also confirmed this
applies to all <.NET Core 3.1 too (because we compile for .NET Standard)


| Method | Runtime | Mean | Allocated | Alloc Ratio |
| -------------------------- | -------------------- | -------: |
--------: | ----------: |
| WriteEnrichedTraces_Before | .NET 6.0 | 488.9 us | 110 B | 0.001 |
| WriteEnrichedTraces_Before | .NET Framework 4.7.2 | 703.3 us | 112537
B | 1.000 |
| | | | | |
| WriteEnrichedTraces_After | .NET 6.0 | 469.1 us | 105 B | 0.50 |
| WriteEnrichedTraces_After | .NET Framework 4.7.2 | 703.4 us | 208 B |
1.00 |

## Implementation details

The fix is to just do what we were doing before #5298 introduced this
regression 😄 i.e. generate code like this:

```csharp
#if NETCOREAPP
    private static ReadOnlySpan<byte> DbTypeBytes => new byte[] { 167, 100, 98, 46, 116, 121, 112, 101 };
#else
    private static readonly byte[] DbTypeBytes = new byte[] { 167, 100, 98, 46, 116, 121, 112, 101 
#endif
```

## Test coverage

This is all covered by existing tests, and the new benchmark shows the
improvment

## Other details

I found a couple of other places in the vendored code that has the same
issue, and fixed them directly in the code. However, this is not ideal,
as if we re-vendor, we'll clobber these updates, so we'll need to update
the vendoring code too

Author: andrewlock

tracer/src/Datadog.Trace.SourceGenerators/TagsListGenerator/Sources.cs

@@ -88,12 +88,24 @@ partial class ")
                       .Append(@""");");
 
                     sb.Append(
-                            @"
+                           @"
+#if NETCOREAPP
         private static ReadOnlySpan<byte> ")
-                        .Append(property.PropertyName)
-                        .Append(@"Bytes => new byte[] { ")
-                        .Append(tagByteArray)
-                        .Append(@" };");
+                      .Append(property.PropertyName)
+                      .Append(@"Bytes => new byte[] { ")
+                      .Append(tagByteArray)
+                      .Append(@" };")
+                      .Append(
+                           @"
+#else
+        private static readonly byte[] ")
+                      .Append(property.PropertyName)
+                      .Append(@"Bytes = new byte[] { ")
+                      .Append(tagByteArray)
+                      .Append(@" };")
+                      .Append(
+                           @"
+#endif");
                 }
             }
 
@@ -112,12 +124,24 @@ partial class ")
                       .Append(@""");");
 
                     sb.Append(
-                            @"
+                           @"
+#if NETCOREAPP
         private static ReadOnlySpan<byte> ")
-                        .Append(property.PropertyName)
-                        .Append(@"Bytes => new byte[] { ")
-                        .Append(tagByteArray)
-                        .Append(@" };");
+                      .Append(property.PropertyName)
+                      .Append(@"Bytes => new byte[] { ")
+                      .Append(tagByteArray)
+                      .Append(@" };")
+                      .Append(
+                           @"
+#else
+        private static readonly byte[] ")
+                      .Append(property.PropertyName)
+                      .Append(@"Bytes = new byte[] { ")
+                      .Append(tagByteArray)
+                      .Append(@" };")
+                      .Append(
+                           @"
+#endif");
                 }
 
                 sb.Append(

tracer/src/Datadog.Trace/Generated/net461/Datadog.Trace.SourceGenerators/TagListGenerator/AerospikeTags.g.cs

@@ -15,17 +15,41 @@ namespace Datadog.Trace.Tagging
     partial class AerospikeTags
     {
         // SpanKindBytes = MessagePack.Serialize("span.kind");
+#if NETCOREAPP
         private static ReadOnlySpan<byte> SpanKindBytes => new byte[] { 169, 115, 112, 97, 110, 46, 107, 105, 110, 100 };
+#else
+        private static readonly byte[] SpanKindBytes = new byte[] { 169, 115, 112, 97, 110, 46, 107, 105, 110, 100 };
+#endif
         // InstrumentationNameBytes = MessagePack.Serialize("component");
+#if NETCOREAPP
         private static ReadOnlySpan<byte> InstrumentationNameBytes => new byte[] { 169, 99, 111, 109, 112, 111, 110, 101, 110, 116 };
+#else
+        private static readonly byte[] InstrumentationNameBytes = new byte[] { 169, 99, 111, 109, 112, 111, 110, 101, 110, 116 };
+#endif
         // KeyBytes = MessagePack.Serialize("aerospike.key");
+#if NETCOREAPP
         private static ReadOnlySpan<byte> KeyBytes => new byte[] { 173, 97, 101, 114, 111, 115, 112, 105, 107, 101, 46, 107, 101, 121 };
+#else
+        private static readonly byte[] KeyBytes = new byte[] { 173, 97, 101, 114, 111, 115, 112, 105, 107, 101, 46, 107, 101, 121 };
+#endif
         // NamespaceBytes = MessagePack.Serialize("aerospike.namespace");
+#if NETCOREAPP
         private static ReadOnlySpan<byte> NamespaceBytes => new byte[] { 179, 97, 101, 114, 111, 115, 112, 105, 107, 101, 46, 110, 97, 109, 101, 115, 112, 97, 99, 101 };
+#else
+        private static readonly byte[] NamespaceBytes = new byte[] { 179, 97, 101, 114, 111, 115, 112, 105, 107, 101, 46, 110, 97, 109, 101, 115, 112, 97, 99, 101 };
+#endif
         // SetNameBytes = MessagePack.Serialize("aerospike.setname");
+#if NETCOREAPP
         private static ReadOnlySpan<byte> SetNameBytes => new byte[] { 177, 97, 101, 114, 111, 115, 112, 105, 107, 101, 46, 115, 101, 116, 110, 97, 109, 101 };
+#else
+        private static readonly byte[] SetNameBytes = new byte[] { 177, 97, 101, 114, 111, 115, 112, 105, 107, 101, 46, 115, 101, 116, 110, 97, 109, 101 };
+#endif
         // UserKeyBytes = MessagePack.Serialize("aerospike.userkey");
+#if NETCOREAPP
         private static ReadOnlySpan<byte> UserKeyBytes => new byte[] { 177, 97, 101, 114, 111, 115, 112, 105, 107, 101, 46, 117, 115, 101, 114, 107, 101, 121 };
+#else
+        private static readonly byte[] UserKeyBytes = new byte[] { 177, 97, 101, 114, 111, 115, 112, 105, 107, 101, 46, 117, 115, 101, 114, 107, 101, 121 };
+#endif
 
         public override string? GetTag(string key)
         {

tracer/src/Datadog.Trace/Generated/net461/Datadog.Trace.SourceGenerators/TagListGenerator/AerospikeV1Tags.g.cs

@@ -15,9 +15,17 @@ namespace Datadog.Trace.Tagging
     partial class AerospikeV1Tags
     {
         // PeerServiceBytes = MessagePack.Serialize("peer.service");
+#if NETCOREAPP
         private static ReadOnlySpan<byte> PeerServiceBytes => new byte[] { 172, 112, 101, 101, 114, 46, 115, 101, 114, 118, 105, 99, 101 };
+#else
+        private static readonly byte[] PeerServiceBytes = new byte[] { 172, 112, 101, 101, 114, 46, 115, 101, 114, 118, 105, 99, 101 };
+#endif
         // PeerServiceSourceBytes = MessagePack.Serialize("_dd.peer.service.source");
+#if NETCOREAPP
         private static ReadOnlySpan<byte> PeerServiceSourceBytes => new byte[] { 183, 95, 100, 100, 46, 112, 101, 101, 114, 46, 115, 101, 114, 118, 105, 99, 101, 46, 115, 111, 117, 114, 99, 101 };
+#else
+        private static readonly byte[] PeerServiceSourceBytes = new byte[] { 183, 95, 100, 100, 46, 112, 101, 101, 114, 46, 115, 101, 114, 118, 105, 99, 101, 46, 115, 111, 117, 114, 99, 101 };
+#endif
 
         public override string? GetTag(string key)
         {

tracer/src/Datadog.Trace/Generated/net461/Datadog.Trace.SourceGenerators/TagListGenerator/AspNetCoreEndpointTags.g.cs

@@ -15,7 +15,11 @@ namespace Datadog.Trace.Tagging
     partial class AspNetCoreEndpointTags
     {
         // AspNetCoreEndpointBytes = MessagePack.Serialize("aspnet_core.endpoint");
+#if NETCOREAPP
         private static ReadOnlySpan<byte> AspNetCoreEndpointBytes => new byte[] { 180, 97, 115, 112, 110, 101, 116, 95, 99, 111, 114, 101, 46, 101, 110, 100, 112, 111, 105, 110, 116 };
+#else
+        private static readonly byte[] AspNetCoreEndpointBytes = new byte[] { 180, 97, 115, 112, 110, 101, 116, 95, 99, 111, 114, 101, 46, 101, 110, 100, 112, 111, 105, 110, 116 };
+#endif
 
         public override string? GetTag(string key)
         {

tracer/src/Datadog.Trace/Generated/net461/Datadog.Trace.SourceGenerators/TagListGenerator/AspNetCoreMvcTags.g.cs

@@ -15,13 +15,29 @@ namespace Datadog.Trace.Tagging
     partial class AspNetCoreMvcTags
     {
         // AspNetCoreControllerBytes = MessagePack.Serialize("aspnet_core.controller");
+#if NETCOREAPP
         private static ReadOnlySpan<byte> AspNetCoreControllerBytes => new byte[] { 182, 97, 115, 112, 110, 101, 116, 95, 99, 111, 114, 101, 46, 99, 111, 110, 116, 114, 111, 108, 108, 101, 114 };
+#else
+        private static readonly byte[] AspNetCoreControllerBytes = new byte[] { 182, 97, 115, 112, 110, 101, 116, 95, 99, 111, 114, 101, 46, 99, 111, 110, 116, 114, 111, 108, 108, 101, 114 };
+#endif
         // AspNetCoreActionBytes = MessagePack.Serialize("aspnet_core.action");
+#if NETCOREAPP
         private static ReadOnlySpan<byte> AspNetCoreActionBytes => new byte[] { 178, 97, 115, 112, 110, 101, 116, 95, 99, 111, 114, 101, 46, 97, 99, 116, 105, 111, 110 };
+#else
+        private static readonly byte[] AspNetCoreActionBytes = new byte[] { 178, 97, 115, 112, 110, 101, 116, 95, 99, 111, 114, 101, 46, 97, 99, 116, 105, 111, 110 };
+#endif
         // AspNetCoreAreaBytes = MessagePack.Serialize("aspnet_core.area");
+#if NETCOREAPP
         private static ReadOnlySpan<byte> AspNetCoreAreaBytes => new byte[] { 176, 97, 115, 112, 110, 101, 116, 95, 99, 111, 114, 101, 46, 97, 114, 101, 97 };
+#else
+        private static readonly byte[] AspNetCoreAreaBytes = new byte[] { 176, 97, 115, 112, 110, 101, 116, 95, 99, 111, 114, 101, 46, 97, 114, 101, 97 };
+#endif
         // AspNetCorePageBytes = MessagePack.Serialize("aspnet_core.page");
+#if NETCOREAPP
         private static ReadOnlySpan<byte> AspNetCorePageBytes => new byte[] { 176, 97, 115, 112, 110, 101, 116, 95, 99, 111, 114, 101, 46, 112, 97, 103, 101 };
+#else
+        private static readonly byte[] AspNetCorePageBytes = new byte[] { 176, 97, 115, 112, 110, 101, 116, 95, 99, 111, 114, 101, 46, 112, 97, 103, 101 };
+#endif
 
         public override string? GetTag(string key)
         {

tracer/src/Datadog.Trace/Generated/net461/Datadog.Trace.SourceGenerators/TagListGenerator/AspNetCoreTags.g.cs

@@ -15,11 +15,23 @@ namespace Datadog.Trace.Tagging
     partial class AspNetCoreTags
     {
         // InstrumentationNameBytes = MessagePack.Serialize("component");
+#if NETCOREAPP
         private static ReadOnlySpan<byte> InstrumentationNameBytes => new byte[] { 169, 99, 111, 109, 112, 111, 110, 101, 110, 116 };
+#else
+        private static readonly byte[] InstrumentationNameBytes = new byte[] { 169, 99, 111, 109, 112, 111, 110, 101, 110, 116 };
+#endif
         // AspNetCoreRouteBytes = MessagePack.Serialize("aspnet_core.route");
+#if NETCOREAPP
         private static ReadOnlySpan<byte> AspNetCoreRouteBytes => new byte[] { 177, 97, 115, 112, 110, 101, 116, 95, 99, 111, 114, 101, 46, 114, 111, 117, 116, 101 };
+#else
+        private static readonly byte[] AspNetCoreRouteBytes = new byte[] { 177, 97, 115, 112, 110, 101, 116, 95, 99, 111, 114, 101, 46, 114, 111, 117, 116, 101 };
+#endif
         // HttpRouteBytes = MessagePack.Serialize("http.route");
+#if NETCOREAPP
         private static ReadOnlySpan<byte> HttpRouteBytes => new byte[] { 170, 104, 116, 116, 112, 46, 114, 111, 117, 116, 101 };
+#else
+        private static readonly byte[] HttpRouteBytes = new byte[] { 170, 104, 116, 116, 112, 46, 114, 111, 117, 116, 101 };
+#endif
 
         public override string? GetTag(string key)
         {

tracer/src/Datadog.Trace/Generated/net461/Datadog.Trace.SourceGenerators/TagListGenerator/AspNetTags.g.cs

@@ -15,17 +15,41 @@ namespace Datadog.Trace.Tagging
     partial class AspNetTags
     {
         // AspNetRouteBytes = MessagePack.Serialize("aspnet.route");
+#if NETCOREAPP
         private static ReadOnlySpan<byte> AspNetRouteBytes => new byte[] { 172, 97, 115, 112, 110, 101, 116, 46, 114, 111, 117, 116, 101 };
+#else
+        private static readonly byte[] AspNetRouteBytes = new byte[] { 172, 97, 115, 112, 110, 101, 116, 46, 114, 111, 117, 116, 101 };
+#endif
         // AspNetControllerBytes = MessagePack.Serialize("aspnet.controller");
+#if NETCOREAPP
         private static ReadOnlySpan<byte> AspNetControllerBytes => new byte[] { 177, 97, 115, 112, 110, 101, 116, 46, 99, 111, 110, 116, 114, 111, 108, 108, 101, 114 };
+#else
+        private static readonly byte[] AspNetControllerBytes = new byte[] { 177, 97, 115, 112, 110, 101, 116, 46, 99, 111, 110, 116, 114, 111, 108, 108, 101, 114 };
+#endif
         // AspNetActionBytes = MessagePack.Serialize("aspnet.action");
+#if NETCOREAPP
         private static ReadOnlySpan<byte> AspNetActionBytes => new byte[] { 173, 97, 115, 112, 110, 101, 116, 46, 97, 99, 116, 105, 111, 110 };
+#else
+        private static readonly byte[] AspNetActionBytes = new byte[] { 173, 97, 115, 112, 110, 101, 116, 46, 97, 99, 116, 105, 111, 110 };
+#endif
         // AspNetAreaBytes = MessagePack.Serialize("aspnet.area");
+#if NETCOREAPP
         private static ReadOnlySpan<byte> AspNetAreaBytes => new byte[] { 171, 97, 115, 112, 110, 101, 116, 46, 97, 114, 101, 97 };
+#else
+        private static readonly byte[] AspNetAreaBytes = new byte[] { 171, 97, 115, 112, 110, 101, 116, 46, 97, 114, 101, 97 };
+#endif
         // HttpRouteBytes = MessagePack.Serialize("http.route");
+#if NETCOREAPP
         private static ReadOnlySpan<byte> HttpRouteBytes => new byte[] { 170, 104, 116, 116, 112, 46, 114, 111, 117, 116, 101 };
+#else
+        private static readonly byte[] HttpRouteBytes = new byte[] { 170, 104, 116, 116, 112, 46, 114, 111, 117, 116, 101 };
+#endif
         // InstrumentationNameBytes = MessagePack.Serialize("component");
+#if NETCOREAPP
         private static ReadOnlySpan<byte> InstrumentationNameBytes => new byte[] { 169, 99, 111, 109, 112, 111, 110, 101, 110, 116 };
+#else
+        private static readonly byte[] InstrumentationNameBytes = new byte[] { 169, 99, 111, 109, 112, 111, 110, 101, 110, 116 };
+#endif
 
         public override string? GetTag(string key)
         {

tracer/src/Datadog.Trace/Generated/net461/Datadog.Trace.SourceGenerators/TagListGenerator/AwsDynamoDbTags.g.cs

@@ -15,9 +15,17 @@ namespace Datadog.Trace.Tagging
     partial class AwsDynamoDbTags
     {
         // TableNameBytes = MessagePack.Serialize("tablename");
+#if NETCOREAPP
         private static ReadOnlySpan<byte> TableNameBytes => new byte[] { 169, 116, 97, 98, 108, 101, 110, 97, 109, 101 };
+#else
+        private static readonly byte[] TableNameBytes = new byte[] { 169, 116, 97, 98, 108, 101, 110, 97, 109, 101 };
+#endif
         // SpanKindBytes = MessagePack.Serialize("span.kind");
+#if NETCOREAPP
         private static ReadOnlySpan<byte> SpanKindBytes => new byte[] { 169, 115, 112, 97, 110, 46, 107, 105, 110, 100 };
+#else
+        private static readonly byte[] SpanKindBytes = new byte[] { 169, 115, 112, 97, 110, 46, 107, 105, 110, 100 };
+#endif
 
         public override string? GetTag(string key)
         {

tracer/src/Datadog.Trace/Generated/net461/Datadog.Trace.SourceGenerators/TagListGenerator/AwsEventBridgeTags.g.cs

@@ -15,9 +15,17 @@ namespace Datadog.Trace.Tagging
     partial class AwsEventBridgeTags
     {
         // RuleNameBytes = MessagePack.Serialize("rulename");
+#if NETCOREAPP
         private static ReadOnlySpan<byte> RuleNameBytes => new byte[] { 168, 114, 117, 108, 101, 110, 97, 109, 101 };
+#else
+        private static readonly byte[] RuleNameBytes = new byte[] { 168, 114, 117, 108, 101, 110, 97, 109, 101 };
+#endif
         // SpanKindBytes = MessagePack.Serialize("span.kind");
+#if NETCOREAPP
         private static ReadOnlySpan<byte> SpanKindBytes => new byte[] { 169, 115, 112, 97, 110, 46, 107, 105, 110, 100 };
+#else
+        private static readonly byte[] SpanKindBytes = new byte[] { 169, 115, 112, 97, 110, 46, 107, 105, 110, 100 };
+#endif
 
         public override string? GetTag(string key)
         {

tracer/src/Datadog.Trace/Generated/net461/Datadog.Trace.SourceGenerators/TagListGenerator/AwsKinesisTags.g.cs

@@ -15,9 +15,17 @@ namespace Datadog.Trace.Tagging
     partial class AwsKinesisTags
     {
         // StreamNameBytes = MessagePack.Serialize("streamname");
+#if NETCOREAPP
         private static ReadOnlySpan<byte> StreamNameBytes => new byte[] { 170, 115, 116, 114, 101, 97, 109, 110, 97, 109, 101 };
+#else
+        private static readonly byte[] StreamNameBytes = new byte[] { 170, 115, 116, 114, 101, 97, 109, 110, 97, 109, 101 };
+#endif
         // SpanKindBytes = MessagePack.Serialize("span.kind");
+#if NETCOREAPP
         private static ReadOnlySpan<byte> SpanKindBytes => new byte[] { 169, 115, 112, 97, 110, 46, 107, 105, 110, 100 };
+#else
+        private static readonly byte[] SpanKindBytes = new byte[] { 169, 115, 112, 97, 110, 46, 107, 105, 110, 100 };
+#endif
 
         public override string? GetTag(string key)
         {