Remove warning about non-serializable test data (#6102)

## Summary of changes

Removes warnings about non-serializable test data

```
Datadog.Trace.Tests: Non-serializable data ('System.Object[]') found for 'Datadog.Trace.Tests.Tagging.ActivityTagsTests.Tags_ShouldBe_PlacedInMetricsOrMeta'; falling back to single test case.
```

## Reason for change

Warnings about `Non-serializable data ('System.Object[]')`.

## Implementation details

Ignored them by setting `DisableDiscoveryEnumeration = true` on
`MemberData`.

## Test coverage

Ran tests locally, warning gone.

## Other details

Maybe I could've changed them to have serializable test data 🤷 
<!-- Fixes #{issue} -->

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tracer/test/Datadog.Trace.Tests/Agent/MessagePack/SpanMetaStructTests.cs

@@ -103,7 +103,7 @@ public class SpanMetaStructTests
             }
        };
 
-    [MemberData(nameof(Data))]
+    [MemberData(nameof(Data), DisableDiscoveryEnumeration = true)]
     [Theory]
     public static void GivenAEncodedSpanWithMetaStruct_WhenDecoding_ThenMetaStructIsCorrectlyDecoded(List<Tuple<string, object?>> dataToEncode)
     {

tracer/test/Datadog.Trace.Tests/Tagging/ActivityTagsTests.cs

@@ -70,7 +70,7 @@ public enum TagKind
     // TODO what about an array of object that contains string and numeric objects?
 
     [Theory]
-    [MemberData(nameof(TagData))]
+    [MemberData(nameof(TagData), DisableDiscoveryEnumeration = true)]
     public void Tags_ShouldBe_PlacedInMetricsOrMeta(string tagKey, object tagValue, TagKind expectedTagKind)
     {
         var activityMock = new Mock<IActivity5>();
@@ -97,7 +97,7 @@ public void Tags_ShouldBe_PlacedInMetricsOrMeta(string tagKey, object tagValue,
     }
 
     [Theory]
-    [MemberData(nameof(ArrayTagData))]
+    [MemberData(nameof(ArrayTagData), DisableDiscoveryEnumeration = true)]
     public void ArrayedTags_ShouldBe_PlacedInMeta(string tagKey, object tagValue, TagKind expectedTagKind, Dictionary<string, object> expectedTagValues)
     {
         var activityMock = new Mock<IActivity5>();