use span-based api when exporting metric infos

[lust4life]

I'm not going to argue here at all, but we should consider using netcoreapp2.2 in the future, and defaulting to the non-allocating Span-based API:s.

_Originally posted by @haf in https://github.com/_render_node/MDE3OlB1bGxSZXF1ZXN0UmV2aWV3MjE0NjgzNzAx/pull_request_reviews/more_threads_

[lust4life]

seems right now, the span-based api in StringBuilder doesn't give more good here. since what we do is format data to string then append to stringbuilder, then generate bytes to suave's response content.

maybe these would help :

• https://github.com/dotnet/corefx/blob/master/src/Common/src/CoreLib/System/Text/ValueStringBuilder.cs
• https://github.com/dotnet/corefxlab/blob/master/src/System.Buffers.ReaderWriter/System/Buffers/Writer/BufferWriter.cs
• https://github.com/dotnet/corefx/issues/28379
• https://github.com/dotnet/csharplang/issues/2302
• https://github.com/dotnet/corefxlab/pull/2595

After these have been done and expose, I think we can make it easier to get the benefit. What do you think ?

[haf]

I think that if we use the Span based API, I can replace the server from underneath to one that supports writing straight to the socket in the future.

I don't think we'll get much benefit immediately though.