Reduce heap allocations in AspNetCoreDiagnosticObserver

[kevingosse]

• Use a wrapper on top of IHeaderDictionary instead of duplicating the headers
• Make all methods that expect a IHeadersCollection generic, to avoid boxing the wrapper
• Cache the SamplingPriority values in a dictionary to avoid the costly enum conversion
• Remove the ToList when accessing the header values

Benchmark:

AspNetCoreDiagnosticObserver.ExtractPropagatedContext(request)

• No headers: request with empty headers
• All headers: request with headers TraceId / ParentId / SamplingPriority

BenchmarkDotNet=v0.12.0, OS=Windows 10.0.19041
Intel Core i7-9750H CPU 2.60GHz, 1 CPU, 12 logical and 6 physical cores
.NET Core SDK=3.1.301
  [Host]     : .NET Core 3.1.5 (CoreCLR 4.700.20.26901, CoreFX 4.700.20.27001), X64 RyuJIT
  DefaultJob : .NET Core 3.1.5 (CoreCLR 4.700.20.26901, CoreFX 4.700.20.27001), X64 RyuJIT

Method	Mean	Error	StdDev	Gen 0	Gen 1	Gen 2	Allocated
NoHeaders_Old	95.84 ns	1.299 ns	1.215 ns	0.0305	-	-	192 B
NoHeaders_New	32.65 ns	0.177 ns	0.147 ns	0.0038	-	-	24 B
AllHeaders_Old	998.29 ns	21.315 ns	19.938 ns	0.1640	-	-	1032 B
AllHeaders_New	226.51 ns	5.037 ns	6.186 ns	0.0443	-	-	280 B

[tonyredondo]

This ToArray() allocates another string array and copy the content (https://source.dot.net/#System.Linq/EnumerableHelpers.Linq.cs,105) maybe we can avoid it by using:

Suggested change

	return values.ToArray();
	return values as string[] ?? values.ToArray();

what do you think?

[kevingosse]

This is StringValues.ToArray(), not Enumerable.ToArray() (confusing, I know)
https://source.dot.net/#Microsoft.Extensions.Primitives/StringValues.cs,280

[tonyredondo]

ok, yeah didn't realize that, this struct is only used by the HttpRequest.Headers

[lucaspimentel]

I like this much better, thanks!

[zacharycmontoya]

Nice!