Remove AllowUnsafeBlocks, MemoryMarshal and Unsafe

[martincostello]

Context: #7307 (comment)

opentelemetry-dotnet/src/OpenTelemetry.Exporter.OpenTelemetryProtocol/OpenTelemetry.Exporter.OpenTelemetryProtocol.csproj

Line 21 in b1dd8e8

<AllowUnsafeBlocks Condition="'$(TargetFramework)' == 'netstandard2.0' OR '$(TargetFramework)' == '$(NetFrameworkMinimumSupportedVersion)'">true</AllowUnsafeBlocks>

opentelemetry-dotnet/src/OpenTelemetry.Exporter.OpenTelemetryProtocol/Implementation/Serializer/ProtobufSerializer.cs

Line 342 in e5efcb6

=> ref (span.Length != 0) ? ref Unsafe.AsRef(in MemoryMarshal.GetReference(span)) : ref Unsafe.AsRef<T>((void*)1);

The OTLP exporter uses unsafe for .NET Standard and .NET Framework for performance reasons.

We also use MemoryMarshal and Unsafe in two places in OpenTelemetry for netX.0 targets:

opentelemetry-dotnet/src/OpenTelemetry/Metrics/Tags.cs

Lines 50 to 51 in e5efcb6

	ref var ours = ref MemoryMarshal.GetArrayDataReference(ourKvps);
	ref var theirs = ref MemoryMarshal.GetArrayDataReference(theirKvps);

opentelemetry-dotnet/src/OpenTelemetry/Metrics/Tags.cs

Lines 69 to 70 in 34dcd84

	ours = ref Unsafe.Add(ref ours, 1);
	theirs = ref Unsafe.Add(ref theirs, 1);

C# 15 16 in the .NET 11 12 timeframe is looking to make changes to unsafe code to reduce its usage for security reasons, see Unsafe Evolution for further context.

To improve the security posture of the OTLP exporter, we should remove the remaining usages of unsafe code even in the face of potential performance degradation.

A similar attempt was made previously in the core library for the same reason: #6594

This may take a while to achieve as if the degradation is large, we may have to make other changes elsewhere to compensate.

The MemoryMarshal and Unsafe usages may get surfaced in later previews of .NET 11 in #6899.

[martincostello]

Improving C# Memory Safety

[unsafePtr]

To improve the security posture of the OTLP exporter, we should remove the remaining usages of unsafe code even in the face of potential performance degradation.

Thought it's more about double-checking the places where unsafe is used. If possible reduce the surface of unsafe or limit it only to specific sections.

[unsafePtr]

For example within this #7307, it's removing unsafe usage from a common path. ProtobufSerializer.cs had fixed/byte* for Encoding.GetBytes on legacy frameworks. I've removed it entirely and routed it through EncodingExtensions.GetBytes(ReadOnlySpan<char>, Span<byte>) which should improve readability and safety.