[mux] Add request filters like otelhttp

[RangelReale]

Based on the otelhttp code.

Fixes #896

[codecov]

Codecov Report

Merging #4230 (fa42803) into main (3ad5a2c) will increase coverage by 0.0%.
The diff coverage is 100.0%.

@@          Coverage Diff          @@
##            main   #4230   +/-   ##
=====================================
  Coverage   79.4%   79.4%           
=====================================
  Files        166     166           
  Lines      10363   10374   +11     
=====================================
+ Hits        8233    8244   +11     
  Misses      1996    1996           
  Partials     134     134           

Files Changed	Coverage
...mentation/github.com/gorilla/mux/otelmux/config.go	100.0%
...trumentation/github.com/gorilla/mux/otelmux/mux.go	100.0%

[pellared]

@RangelReale Thank you for your contribution 🎉

FYI #2834 (comment)

It is possible that in the future we will deprecate most HTTP instrumentation libraries in favor of having otelhttp in better shape and more extensible. We for should would add examples how to use it with libraries like mux or chi.