[Service Disc] Change type layout to make it x86 safe (#7387)

## Summary of changes

Change service discovery result type to work in x86 systems.

## Reason for change

tested locally removing x64 guards it works on  both

## Implementation details

use a  mix of sequential and explicit layouts

## Test coverage

## Other details
<!-- Fixes #{issue} -->


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Author: anna-git

tracer/src/Datadog.Trace/LibDatadog/ServiceDiscovery/ServiceDiscoveryHelper.cs

@@ -42,8 +42,8 @@ internal static StoreMetadataResult StoreTracerMetadata(TracerSettings tracerSet
 
                 if (result.Tag == ResultTag.Error)
                 {
-                    Log.Error("Failed to store tracer metadata with message: {Error}", result.Error.Message.ToUtf8String());
-                    NativeInterop.Common.DropError(ref result.Error);
+                    Log.Error("Failed to store tracer metadata with message: {Error}", result.Result.Error.Message.ToUtf8String());
+                    NativeInterop.Common.DropError(ref result.Result.Error);
                     return StoreMetadataResult.Error;
                 }
 

tracer/src/Datadog.Trace/LibDatadog/ServiceDiscovery/TracerMemfdHandleResult.cs

@@ -10,21 +10,24 @@
 namespace Datadog.Trace.LibDatadog.ServiceDiscovery;
 
 /// <summary>
-/// **DO NOT USE THIS TYPE in x86** to map with Libdatadog. OK and Err fields needs a 4 offset instead.
 /// Do not change the values of this enum unless you really need to update the interop mapping.
 /// Libdatadog interop mapping of the generic type: https://github.com/DataDog/libdatadog/blob/60583218a8de6768f67d04fcd5bc6443f67f516b/ddcommon-ffi/src/result.rs#L44
 /// Cf also ddog_Result_TracerMemfdHandle in common.h headers.
 /// </summary>
-[StructLayout(LayoutKind.Explicit)]
+[StructLayout(LayoutKind.Sequential)]
 internal struct TracerMemfdHandleResult
 {
-    [FieldOffset(0)]
     public ResultTag Tag;
 
-    // beware that offset 8 is only valid on x64 and would cause a crash if read on x86.
-    [FieldOffset(8)]
-    public int TracerMemfdHandle;
+    public ResultUnion Result;
 
-    [FieldOffset(8)]
-    public Error Error;
+    [StructLayout(LayoutKind.Explicit)]
+    public struct ResultUnion
+    {
+        [FieldOffset(0)]
+        public int TracerMemfdHandle;
+
+        [FieldOffset(0)]
+        public Error Error;
+    }
 }