fix: resolved ReadOnlySpan missing

[bitsandfoxes]

Addresses #2751

We excluded those libraries from being aliased in #2726.
This had the side effect of also setting the PublicKeyToken to null. When users would call EmitCounter<T>(string name, T value, ReadOnlySpan<KeyValuePair<string, object>> attributes). this would cause

error CS0012: The type 'ReadOnlySpan<>' is defined in an assembly that is not referenced. You must add a reference to assembly 'System.Memory, Version=4.0.1.2, Culture=neutral, PublicKeyToken=null'.

Since we're already not aliasing those assemblies and we let Unity handle the resolution, we can simply rely on what Unity already comes with.

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[JoshuaMoelans]

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