otelhttp: handle request Pattern

[dmathieu]

Closes #6193
Handles otelhttp for #6919 (@Ananyasinha13 I know you're assigned this, but it has been 2 weeks with no movement).

[dmathieu]

This has the downside of running the span name formatter twice if there's a pattern.
I'm open to alternatives, but it would be good if they weren't modifying the public API.

[dmathieu]

cc @seankhliao @axw

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.1%. Comparing base (c1aeca6) to head (d8965e4).

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #7192   +/-   ##
=====================================
  Coverage   81.1%   81.1%           
=====================================
  Files        204     204           
  Lines      18140   18144    +4     
=====================================
+ Hits       14720   14727    +7     
+ Misses      3001    2999    -2     
+ Partials     419     418    -1     

Files with missing lines	Coverage Δ
instrumentation/net/http/otelhttp/config.go	83.7% <ø> (ø)
instrumentation/net/http/otelhttp/handler.go	93.0% <100.0%> (+0.1%)	⬆️

... and 1 file with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[axw]

Neat.

One other downside of changing the name after handling the request is that it won't work well with samplers that care about the span name, like the per-operation Jaeger remote sampler, IIANM.

[dmathieu]

I can add a comment to the WithSpanNameFormatter method for better awareness of that.

[seankhliao]

It's a cool trick,
it may be a bit fragile if someone uses some other middleware that clones the request (uses WithContext) between otelhttp and the mux, only the new copy will have the Pattern set.

Can we have a spanformatter func or option to use the pattern (a separate pr?)

[dmathieu]

Can we have a spanformatter func or option to use the pattern (a separate pr?)

I'd make that a separate PR, yes. Do you want to open one to better articulate what you need?

[seankhliao]

sure I can make one

[seankhliao]

I was thinking something simple like a44c60f (currently based on top of this pr).