Fix bug with multiple views mapping to a single metric stream

[alanwest]

My original implementation from #2916 did not correctly handle some cases when using views.

[codecov]

Codecov Report

Merging #3006 (7b70b30) into main (d294140) will increase coverage by 0.10%.
The diff coverage is 90.10%.

@@            Coverage Diff             @@
##             main    #3006      +/-   ##
==========================================
+ Coverage   84.66%   84.77%   +0.10%     
==========================================
  Files         259      260       +1     
  Lines        9228     9273      +45     
==========================================
+ Hits         7813     7861      +48     
+ Misses       1415     1412       -3     

Impacted Files	Coverage Δ
...enTelemetry/Metrics/StringArrayEqualityComparer.cs	80.00% <80.00%> (ø)
src/OpenTelemetry/Metrics/MetricStreamIdentity.cs	90.00% <90.00%> (ø)
src/OpenTelemetry/Metrics/Metric.cs	95.65% <100.00%> (ø)
src/OpenTelemetry/Metrics/MetricReaderExt.cs	86.06% <100.00%> (+0.11%)	⬆️
...mentation/ExportClient/BaseOtlpGrpcExportClient.cs	61.11% <0.00%> (+11.11%)	⬆️
...entation/ExportClient/OtlpGrpcTraceExportClient.cs	50.00% <0.00%> (+14.28%)	⬆️
...xporter.OpenTelemetryProtocol/OtlpTraceExporter.cs	59.09% <0.00%> (+22.72%)	⬆️

[CodeBlanch]

I don't think we need readonly on all the members because the struct itself is readonly. Any harm in having it? Nothing major that I can think of. Might lead to a bigger binary, I'm assuming readonly ends up a hint in the IL metadata. More code to maintain 😄

[CodeBlanch]

@alanwest I just took a quick look, I think there might be some runtime bits we can use for these array comparers. Check out this fiddle:

using System;
using System.Collections;
using System.Collections.Generic;
					
public class Program
{
	public static void Main()
	{
		string[] first = new string[] { "one", "two", "three" };
		string[] second = new string[] { "one", "two", "three" };
		string[] third = new string[] { "one", "two", "THREE" };
		
		Console.WriteLine(((IStructuralEquatable)first).Equals(second, StringComparer.Ordinal));
		Console.WriteLine(((IStructuralEquatable)first).Equals(third, StringComparer.Ordinal));
		Console.WriteLine(((IStructuralEquatable)first).Equals(third, StringComparer.OrdinalIgnoreCase));
		
		double[] firstD = new double[] { 0.18D, 1D, 0.99D };
		double[] secondD = new double[] { 0.18D, 1D, 0.99D };
		
		Console.WriteLine(((IStructuralEquatable)firstD).Equals(secondD, EqualityComparer<double>.Default));
	}
}

It looks like Array has this hidden (explicitly implemented) IStructuralEquatable we might be able to use.

[alanwest]

Cool I like this. From looking at reference source looks like Array has implemented IStructuralEquatable for quite some time.

[alanwest]

I reverted my change to use IStructuralEquatable. Unit tests were failing only for net461. I haven't dug into why. I'd like to follow up with trying out IStructureEquatable in a separate PR because it definitely does simplify the code.

[github-actions]

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[alanwest]

Merging this. There are a few follow ups coming soon.

• There's still a case this PR is not handling correctly. Namely multiple view configs that generate identical metric streams with this PR are "merged" into one stream. Each view should in fact result in a separate stream.
• Tests from this PR should be moved to MetricViewTests.cs.
• IStructuralEquatable may help simplify some of this code.