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 81.2%. Comparing base (09c7ab5) to head (4ebb00d).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #7192     +/-   ##
=======================================
- Coverage   81.2%   81.2%   -0.1%     
=======================================
  Files        207     207             
  Lines      18271   18275      +4     
=======================================
+ Hits       14844   14845      +1     
- Misses      3006    3008      +2     
- Partials     421     422      +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

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[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).

[MrAlias]

@XSAM PTAL