Setup PublicEndpointFn option

[dmathieu]

Marking all endpoints as public is a pretty good way to avoid orphan spans.
However, this is an all-or-nothing situation, which is rarely accurate. For example, a public API could have both external and internal customers.
External customers' requests need to be considered as public, while internal ones need not to.

This introduces a WithPublicEndpointFn option, which allows setting a specific request as public or not based on the return value of a function.
A specific TraceContext baggage could then be set on all internal requests, to keep the context while discarding it for any request which doesn't have it.

[codecov]

Codecov Report

Merging #2342 (b76c0c6) into main (099c817) will decrease coverage by 0.0%.
The diff coverage is 100.0%.

❗ Current head b76c0c6 differs from pull request most recent head 756c3fd. Consider uploading reports for the commit 756c3fd to get more accurate results

@@           Coverage Diff           @@
##            main   #2342     +/-   ##
=======================================
- Coverage   74.3%   74.2%   -0.1%     
=======================================
  Files        144     144             
  Lines       6563    6569      +6     
=======================================
+ Hits        4877    4880      +3     
- Misses      1543    1546      +3     
  Partials     143     143             

Impacted Files	Coverage Δ
instrumentation/net/http/otelhttp/config.go	87.8% <100.0%> (+0.5%)	⬆️
instrumentation/net/http/otelhttp/handler.go	79.5% <100.0%> (-1.6%)	⬇️

[dashpole]

I can see this being useful for kubernetes/kubernetes#103186 :).

[dmathieu]

There was a blurp in one of the rebases, which made the test code non-parseable. I've rebased from my initial commit (dropping the merge commits).