[pkg/otlp] Disable OTLP internal metrics port

[mx-psi]

What does this PR do?

Disable OTLP Prometheus port on 0.0.0.0:8888, which exposed internal observability metrics of the OTLP pipeline.

Motivation

• Avoid reserving an additional port in all interfaces
• Avoid reserving an additional port that clashes with a running OpenTelemetry Collector
• Avoid exposing a feature that is experimental and will change. We can re-enable this once the Collector switches to using opentelemetry-go for internal metrics, and create a shim exporter that unifies these metrics with other Agent internal metrics.

Additional Notes

This depends on #10652 and #10638.

This is technically a breaking change, but we have never publicly advertised the existance of this port, so I think a release note is more confusing than helpful.

Describe how to test/QA your changes

Run the Datadog Agent with the OTLP ingest feature enabled. Check that the port 8888 is not reserved.

Reviewer's Checklist

• If known, an appropriate milestone has been selected; otherwise the Triage milestone is set.
• The appropriate team/.. label has been applied, if known.
• Use the major_change label if your change either has a major impact on the code base, is impacting multiple teams or is changing important well-established internals of the Agent. This label will be use during QA to make sure each team pay extra attention to the changed behavior. For any customer facing change use a releasenote.
• A release note has been added or the changelog/no-changelog label has been applied.
• Changed code has automated tests for its functionality.
• Adequate QA/testing plan information is provided if the qa/skip-qa label is not applied.
• If applicable, docs team has been notified or an issue has been opened on the documentation repo.
• If applicable, the need-change/operator and need-change/helm labels have been applied.
• If applicable, the config template has been updated.

[albertvaka]

Was this port documented somewhere? We might want to update those docs after releasing this.

[mx-psi]

I don't think we have documented this anywhere. It is documented on the OpenTelemetry Collector docs, but the fact that we use the Collector as a library for OTLP ingest is an implementation detail.