Add support for the WithMetricAttributesFn option for otelhttp middlewares

[TanishqPorwar]

add metricAttributesFn to http server handler in otelhttp, for dynamically setting per-request metric attributes

[dmathieu]

Could you add tests?

[TanishqPorwar]

@dmathieu I have added some tests.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.5%. Comparing base (96acbc0) to head (57dafe0).
Report is 1 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #6542   +/-   ##
=====================================
  Coverage   68.5%   68.5%           
=====================================
  Files        200     200           
  Lines      16772   16781    +9     
=====================================
+ Hits       11491   11500    +9     
  Misses      4935    4935           
  Partials     346     346           

Files with missing lines	Coverage Δ
instrumentation/net/http/otelhttp/handler.go	92.8% <100.0%> (+0.4%)	⬆️