Add cross-module test coverage

[pellared]

Why

Per #897 (comment)

What

Add cross-module test coverage

Details

https://github.com/wadey/gocovmerge is used to merge lines like these:

go.opentelemetry.io/contrib/contrib.go:20.23,23.2 1 4
...
go.opentelemetry.io/contrib/contrib.go:20.23,23.2 1 1
...

into:

go.opentelemetry.io/contrib/contrib.go:20.23,23.2 1 204

Next PR

Do the same for https://github.com/open-telemetry/opentelemetry-go

[codecov]

Codecov Report

Merging #966 (aa51148) into main (be914d3) will increase coverage by 0.1%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##            main    #966     +/-   ##
=======================================
+ Coverage   75.6%   75.7%   +0.1%     
=======================================
  Files         69      70      +1     
  Lines       4460    4467      +7     
=======================================
+ Hits        3372    3382     +10     
+ Misses       946     943      -3     
  Partials     142     142             

Impacted Files	Coverage Δ
...mentation/github.com/gorilla/mux/otelmux/config.go	41.6% <0.0%> (-18.4%)	⬇️
contrib.go	100.0% <0.0%> (ø)
instrumentation/net/http/otelhttp/handler.go	81.1% <0.0%> (+4.1%)	⬆️

[MrAlias]

🎉

[MrAlias]

Planning to merge this before the 1 day policy as it doesn't touch the user facing code, only tooling, and will help give an accurate coverage report for things like #954.