prometheus exporter ignores metrics from the Prometheus bridge

[dashpole]

Related to #7686.

This makes the Prometheus exporter ignore all metrics from the Prometheus bridge. This is almost certainly a misconfiguration, but may happen when using autoexport or other packages that automatically install the bridge.

[codecov]

Codecov Report

❌ Patch coverage is 42.85714% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.5%. Comparing base (18863ca) to head (d3f4651).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
exporters/prometheus/exporter.go	42.8%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #7688     +/-   ##
=======================================
- Coverage   86.1%   85.5%   -0.7%     
=======================================
  Files        298     298             
  Lines      21721   21763     +42     
=======================================
- Hits       18718   18623     -95     
- Misses      2626    2663     +37     
- Partials     377     477    +100     

Files with missing lines	Coverage Δ
exporters/prometheus/exporter.go	63.8% <42.8%> (-26.9%)	⬇️

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[MrAlias]

The idea of these changes makes sense to me. It is something that we do not want to support. I think we need to make it clearer to (new) users what is happening and what to expect upfront.

This behavior, and the justification for it, needs to be documented in the package documentation.

[MrAlias]

Can we update doc.go with details that this exporter is not compatible with the bridge?

[dashpole]

updated