autoexport: Add OTEL_METRICS_PRODUCERS environment variable support

[gouthamve]

This will allow using the Prometheus bridge with autoexport.

The goal is to make it easy for any Prometheus exporter to also be able to emit OTLP. See: https://prometheus.io/blog/2024/03/14/commitment-to-opentelemetry/#otlp-export-in-the-ecosystem

[codecov]

Codecov Report

Attention: Patch coverage is 72.05882% with 19 lines in your changes are missing coverage. Please review.

Project coverage is 62.4%. Comparing base (52576e4) to head (b889ef0).

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #5281   +/-   ##
=====================================
  Coverage   62.3%   62.4%           
=====================================
  Files        189     189           
  Lines      11575   11638   +63     
=====================================
+ Hits        7219    7263   +44     
- Misses      4146    4158   +12     
- Partials     210     217    +7     

Files	Coverage Δ
exporters/autoexport/registry.go	100.0% <100.0%> (ø)
exporters/autoexport/noop.go	62.5% <0.0%> (-20.9%)	⬇️
exporters/autoexport/metrics.go	78.5% <76.1%> (-3.6%)	⬇️

[dashpole]

@gouthamve thanks for this! It will need an entry in CHANGELOG.md

[gouthamve]

Thanks for your review @dashpole!

Anything I can do to move this along @open-telemetry/go-approvers?

[pellared]

@gouthamve Sorry for the delay. I will do my best to review it this week.

[gouthamve]

Thanks for the review @pellared. I did the easiest thing here and exposed the Config object. If you see a better way, please let me know.

Also, this seems a little counter-intuitive to me. We are using the same factory and Config interface for every signal type, but the options I added are only required for the "metric" signals.

[gouthamve]

Hi @pellared @dashpole took a stab at implementing what we discussed? Does this direction look good?

Will add unit tests once you confirm!

[dashpole]

implementation looks correct to me.

[gouthamve]

@pellared @dashpole This is now ready for review!

I added the tests and also a WithFallbackMetricsProducer option. Thanks in advance for your review!