config: Add WithPrometheusRegisterer config option

[codeboten]

This is to allow the OTel Collector to configure its own handler for prometheus metrics, allowing it to manage error reporting and shutdown.

I'm opening this PR to get feedback from contributors on whether this approach makes sense or not, will add tests once I get some feedback.

[codecov]

Codecov Report

Attention: Patch coverage is 43.75000% with 9 lines in your changes missing coverage. Please review.

Project coverage is 65.6%. Comparing base (96fdd2e) to head (111b5b0).
Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
config/config.go	0.0%	5 Missing ⚠️
config/metric.go	63.6%	3 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #6091     +/-   ##
=======================================
- Coverage   65.7%   65.6%   -0.1%     
=======================================
  Files        203     203             
  Lines      13037   13045      +8     
=======================================
  Hits        8568    8568             
- Misses      4206    4213      +7     
- Partials     263     264      +1     

Files with missing lines	Coverage Δ
config/metric.go	90.5% <63.6%> (-1.1%)	⬇️
config/config.go	74.0% <0.0%> (-8.3%)	⬇️

[dashpole]

Can you help explain this a bit more to me? Doesn't this mean most of the prometheus configuration provided via config (e.g. port, path) is ignored? Why does the collector need this behavior?

[codeboten]

So the bulk of the prometheus handler code in the collector exists here: https://github.com/open-telemetry/opentelemetry-collector/blob/e99074da2f6525cd177f26b46b8d03fef2fe4f35/service/internal/proctelemetry/config.go#L106-L116

I'm totally open to suggestions as to a better way to implement this, the problem I was trying to solve was handling errors without passing a channel around.

[dashpole]

I wonder if we should use the otel error handler for the async errors from this package: https://github.com/open-telemetry/opentelemetry-go/blob/29c0c28a2c13f898c8245250aab8f5f336250a91/handler.go#L33. Then also handle those errors generally in the collector however you see fit? What do you do with async errors you get back?

[codeboten]

So the error channel used terminates the collector on error:

    // AsyncErrorChannel is the channel that is used to report fatal errors.
    AsyncErrorChannel chan error

I suppose we could register an error handler and set it to return the error to the channel instead?

[codeboten]

I'll try to implement that and see how far i can get. will put this in draft until I get that working

[codeboten]

Closing, this option is not required as using the otel handler can provide the functionality the collector needed