confighttp: route-based span naming for ServeMux

[axw]

Description

Update confighttp.ServerConfig.ToServer to handle ServeMux specially: if the handler supplied to ToServer is an http.ServeMux, we use its Handler method to determine the matching pattern and use that to name the span as described at https://opentelemetry.io/docs/specs/semconv/http/http-spans/#name. If there is no matching pattern then we fall back to "unknown route".

This approach means that everything will magically just work for components that supply a ServeMux, but there are a couple of downsides:

• this is a little bit magical, and won't help for components that don't happen to use a ServeMux
• route pattern matching needs to be done twice: once for the instrumentation, and once for routing to the correct handler

The alternative that I started working on before I came to this solution was to introduce a new method, ServerConfig.ToServeMux, which would not accept an http.Handler, and would instead return a type that wraps *http.ServeMux, and would instrument handlers as they are registered. With that approach the instrumentation would come after the routing, and we would install a default / NotFoundHandler that is also instrumented. This approach is a bit less magical, and a bit more efficient, but would require all the components to be updated.

Link to tracking issue

Fixes #12468 (for any receiver that uses a ServeMux, like otlpreceiver)

Testing

• Added a unit test to confighttp for the instrumentation added by ServerConfig.ToServer.
• Tested with a otlpreceiver:
  • ran make run with trace telemetry enabled, exporting to console
  • used telemetrygen logs --otlp-http, confirmed that a span named "POST /v1/logs" is produced
  • used curl http://localhost:4318, confirmed that a span named "GET unknown route" is produced

Documentation

Added doc comment to confighttp.ServerConfig.ToServer explaining the new behaviour.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.17%. Comparing base (7d3e03e) to head (52f39b3).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #12593      +/-   ##
==========================================
- Coverage   92.18%   92.17%   -0.02%     
==========================================
  Files         469      471       +2     
  Lines       25394    25583     +189     
==========================================
+ Hits        23409    23580     +171     
- Misses       1574     1591      +17     
- Partials      411      412       +1     

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