Adds benchmark tests for ReadOnlySequence<T>.FirstSpan

[maryamariyan]

Adds benchmark tests for ReadOnlySequence.FirstSpan covering all cases

Related to https://github.com/dotnet/corefx/issues/33029

cc: @adamsitnik @ahsonkhan

[billwert]

I didn't leave a comment on each instance of the new underscores. They should all be fixed. Thanks!

[ahsonkhan]

@billwert, @jorive, I am just curious, is this the expected way to single out API benchmarks that only exist on certain TFMs? What if I want to add benchmarks for types that only exist on netstandard2.0, or only exist on .NET Core 2.0+, etc.?

[billwert]

good question. I don't think we have a solid plan yet. (And this approach is fragile, of course.) @adamsitnik may have considered this - let's see what he says when he's back from vacation.

[adamsitnik]

@ahsonkhan it depends. if it's a single method or just a few like here, you can use #if TFM. If it's an entire type, you should exclude it per certain TFM in the .csproj file

performance/src/benchmarks/micro/MicroBenchmarks.csproj

Lines 60 to 61 in eaec460

	<ItemGroup Condition=" '$(TargetFrameworkIdentifier)' == '.NETFramework' Or '$(TargetFramework)' == 'netcoreapp2.0'">
	<Compile Remove="corefx\System.IO.Compression\Brotli.cs" />

[ahsonkhan]

Sure, I figured that would be the expected approach. That's what I ended up doing for the recent JsonWriter perf tests. Thanks for clarifying.