docs: document Java OTLP export limitations

[arnabnandy7]

Summary

Documents the Java agent’s experimental OTLP Profiles export support in the authoritative Pyroscope documentation.

Changes

• Explain how to enable OTLP export using PYROSCOPE_FORMAT=otlp.
• Document the OTLP/HTTP endpoint used by the Java agent.
• Clarify profiler and profiling-event limitations.
• Warn that application names and labels are currently omitted from OTLP profiles, which may cause profiles to appear under service_name="unknown_service".
• Add otlp to the PYROSCOPE_FORMAT configuration reference.

Related issue

• OTLP export silently drops application name and labels pyroscope-java#344
• fix: warn about ignored OTLP application name and labels pyroscope-java#348

Testing

NA/Documentation updated only

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[arnabnandy7]

@korniltsev @korniltsev-grafanista please review