Support explicit endpoint for Prometheus exporter.

[nluk]

Is your feature request related to a problem? Please describe.

Long story short: Prometheus exporter responds to GET requests on any path/endpoint:

• /metrics
• /liveness
• /my/pets/123
• /

In my case, I didn't notice a missing update in a k8s manifest - custom listener on port 12321 used to handle both metrics and liveness/readiness. I've switched liveness to the application port, and kept 12321 for Prometheus exporter. I forgot to update the probes, and the exporter responded with 200 OK to probe calls on /liveness and /readiness, thus hiding the missconfiguration.

Describe the solution you'd like

New enviromental variable:
OTEL_EXPORTER_PROMETHEUS_PATH=/metrics

or:
OTEL_EXPORTER_PROMETHEUS_PATHS=/metrics,/foo,/

Also, 404 response for requests that do not match the endpoint.

Describe alternatives you've considered

No response

Additional context

No response

[jack-berg]

Moving to opentelemetry-java, which is where there prometheus exporter is defined.

[fstab]

I'm happy to implement this after #6015 is merged, it should be straightforward.

I'm wondering: The Prometheus client_java library had the same issue, and with the 1.x release we changed the default behavior to only export metrics on /metrics.

In the OTel SDK we left the previous behavior, exporting metrics on all HTTP paths. However, we could change the default as well and only export metrics on /metrics. We can still allow to overwrite this for backwards compatibility.

What do you think?

[jack-berg]

What about going in the prometheus client_java direction and only exporting on /metrics? What are the use cases for configuring a different path or multiple paths?

Some data points:

• The [collector prometheus exporter](https://github.com/open-telemetry/opentelemetry-collector-contrib/tree/main/exporter/prometheusexporter endpoint doesn't appears to allow the path to be configured
• The dotnet prometheus exporter does allow the path to be configured
• The go prometheus exporter doesn't appear to allow the path to be configured

Note, to add a new environment variable like OTEL_EXPORTER_PROMETHEUS_PATH, the spec would have to be extended. Making that happen is a longshot because there's a moratorium on adding new environment variables, with the goal to develop / extend file configuration instead.