apm: document otel metrics support in java

[mcculls]

What does this PR do? What is the motivation?

This PR adds OpenTelemetry Metrics API documentation for Java. The goal is to ensure developers have accurate setup and usage instructions for metrics functionality in this language.

Merge instructions

Merge readiness:

• Ready for merge

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Additional notes

[github-actions]

Preview links (active after the build_preview check completes)

Modified Files

• https://docs-staging.datadoghq.com/mcculls/otel-metrics-java/opentelemetry/instrument/dd_sdks/api_support