Avoid allocating a closure for ActivityMappingById.GetOrAdd (#8039)

## Summary of changes

Avoids allocating a closure in .NET Core if we can avoid it

## Reason for change

.NET Core's `ConcurrentDictionary.GetOrAdd()` method allows providing a
"state" object which we can pass to the `GetOrAdd` method. Using this
method avoids allocating a closure every time the method is hit, and we
can pass the state using a value tuple to avoid additional allocation
there

## Implementation details

`#if`/`#else` to glory 

## Test coverage

Functionality is covered by existing tests, benchmarks show an
incremental improvement over #8037 for .NET Core, as expected:


| Method | Runtime | Mean | StdDev | Gen0 | Allocated | Compared to
#8037 |
| ------------------------------- | -------------------- | -------: |
--------: | -----: | --------: | ----------------: |
| StartStopWithChild_Hierarchical | .NET 6.0 | 3.746 us | 0.1128 us |
0.0114 | 3.81 KB | -0.28 KB |
| StartStopWithChild_Hierarchical | .NET 8.0 | 2.759 us | 0.0374 us |
0.0114 | 3.81 KB | -0.28 KB |
| StartStopWithChild_Hierarchical | .NET Core 3.1 | 4.762 us | 0.0584 us
| 0.0153 | 4.04 KB | -0.28 KB |
| StartStopWithChild_Hierarchical | .NET Framework 4.7.2 | 5.651 us |
0.0717 us | 0.7324 | 4.54 KB | 0.01 KB (noise) |
| | | | | | | |
| StartStopWithChild | .NET 6.0 | 3.607 us | 0.0508 us | 0.0153 | 4.02
KB | -0.28 KB |
| StartStopWithChild | .NET 8.0 | 2.921 us | 0.0617 us | 0.0114 | 3.91
KB | -0.29 KB |
| StartStopWithChild | .NET Core 3.1 | 4.922 us | 0.0407 us | 0.0153 |
4.2 KB | -0.28 KB |
| StartStopWithChild | .NET Framework 4.7.2 | 6.008 us | 0.0979 us |
0.7629 | 4.69 KB | 0 |


## Other details

https://datadoghq.atlassian.net/browse/LANGPLAT-915

Part of a stack working to improve OTel performance

- #8036
- #8037
- #8038
- #8039 👈
- #8040
- #8041
- #8042

Author: andrewlock

tracer/src/Datadog.Trace/Activity/Handlers/ActivityHandlerCommon.cs

@@ -176,15 +176,23 @@ public static void ActivityStarted<T>(string sourceName, T activity, OpenTelemet
                     return;
                 }
 
+#if NETCOREAPP
+                // Avoid closure allocation if we can
+                activityMapping = ActivityMappingById.GetOrAdd(
+                    activityKey.Value,
+                    static (_, details) => new(details.activity.Instance!, CreateScopeFromActivity(details.activity, details.tags, details.parent, details.traceId, details.spanId, details.rawTraceId, details.rawSpanId)),
+                    (activity, tags, parent, traceId, spanId, rawTraceId, rawSpanId));
+#else
                 activityMapping = ActivityMappingById.GetOrAdd(activityKey.Value, _ => new(activity.Instance!, CreateScopeFromActivity(activity, tags, parent, traceId, spanId, rawTraceId, rawSpanId)));
+#endif
             }
             catch (Exception ex)
             {
                 Log.Error(ex, "Error processing the OnActivityStarted callback");
                 activityMapping = default;
             }
 
-            static Scope CreateScopeFromActivity(T activity, ITags? tags, SpanContext? parent, TraceId traceId, ulong spanId, string? rawTraceId, string? rawSpanId)
+            static Scope CreateScopeFromActivity(T activity, OpenTelemetryTags? tags, SpanContext? parent, TraceId traceId, ulong spanId, string? rawTraceId, string? rawSpanId)
             {
                 var span = Tracer.Instance.StartSpan(
                     activity.OperationName,