Fix fetching Go server status code

[grcevski]

Our status code fetching for Go HTTP/HTTP2 was relying on a single interface implementation for the response writer. This works in most cases, but it doesn't work with custom response writers, which can be implemented by users and as I recently discovered, the GRPC server has another mode, which lets users implement a single handler for HTTP and gRPC.

This PR does few things:

1. Instead of digging into the response object, we rely on the write status function to capture the status. This is better because we control which implementations we know of. HTTP/HTTP2 standard cases work now and if it's unknown implementation that doesn't delegate to the underlying HTTP/HTTP2, the worst that will happen is status 0, instead of getting high cardinality garbage value.
2. I fixed the negative content size. In Go we read the content size from the headers, and perhaps surprisingly, this value can be -1. Fixes Crash in prom exporter with negative increment #1027. For example, my new test for GRPC in plaintext mode captures this HTTP2 wrapper (with size -1):

2024-07-21 19:48:40.72174840 (2.805882ms[2.597025ms]) SRV 200 PRI * [172.18.0.5 as 172.18.0.5:46108]->[172.18.0.4 as 172.18.0.4:8080] size:-1B svc=[grpc-http2-go/server go] traceparent=[00-b60d354b576686820357bd40ec4bddc2-0000000000000000-01]

I confirmed in the headers the value is indeed -1.

4. I added a test for this new GRPC/HTTP2 Mux approach, for both plaintext and TLS.
5. I cleaned up some stale unused Go offsets we had.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.59%. Comparing base (9a5f90e) to head (ac22c13).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1044      +/-   ##
==========================================
+ Coverage   73.32%   81.59%   +8.26%     
==========================================
  Files         137      138       +1     
  Lines       11014    11028      +14     
==========================================
+ Hits         8076     8998     +922     
+ Misses       2306     1514     -792     
+ Partials      632      516     -116     

Flag	Coverage Δ
integration-test	56.72% <100.00%> (+0.07%)	⬆️
k8s-integration-test	58.93% <86.66%> (-0.27%)	⬇️
oats-test	37.40% <46.66%> (+0.01%)	⬆️
unittests	50.85% <53.33%> (?)

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[mariomac]

Current implementation is even more elegant and concise than previous one. Congrats and thanks!

[grcevski]

Thanks @mariomac !