[extension/prometheusapiserverextension] feat: implement Prometheus API extension

[gracewehner]

Description: Add Prometheus API as an extension for debugging purposes to be able to see the targets, config, and service discovery for what the prometheus receiver is scraping. This uses the Prometheus API packages in order to duplicate less code. It follows the same design to host the server as the Prometheus UI web handler.

Link to tracking Issue: Issue: https://github.com/open-telemetry/wg-prometheus/issues/63 Relevant PR discussions:

• https://github.com/open-telemetry/opentelemetry-collector-contrib/pull/29622
• https://github.com/open-telemetry/opentelemetry-collector-contrib/pull/23244

Testing: Ran an otelcollector with prometheusreceiver and extension. Added tests to ensure all expected paths in the API are working and all go routines are cleaned up

Documentation: Added to READMEs for extension and prometheusreceiver

[gracewehner]

Thanks @Aneurysm9 and @dashpole, I have made the changes we discussed. Please let me know if I should revert this back to all being inside the prometheus receiver.

This PR is also dependent on https://github.com/prometheus/prometheus/pull/13932 to be merged first.

[dashpole]

Please let me know if I should revert this back to all being inside the prometheus receiver.

I think that is what we had decided in the last meeting, right?

[github-actions[bot]]

This PR was marked stale due to lack of activity. It will be closed in 14 days.

[dashpole]

Looks like the promethues PR linked above was merged. Do we need to wait for a release of prometheus/prometheus?

[github-actions[bot]]

This PR was marked stale due to lack of activity. It will be closed in 14 days.

[github-actions[bot]]

Closed as inactive. Feel free to reopen if this PR is still being worked on.