fix: warn about ignored OTLP application name and labels

[arnabnandy7]

Documents the current OTLP export limitation and warns users that the configured application name and labels are not included in OTLP profiles. These profiles may therefore appear under service_name="unknown_service".

It also clarifies why LabelsWrapper.dump() is still called in OTLP mode: besides producing the labels snapshot, it removes closed scoped contexts and prevents them from accumulating.

Changes

• Document the OTLP application name and label limitation in the README.
• Log a warning at startup when OTLP export is enabled.
• Add tests covering the warning.
• Isolate PyroscopeAgentTest from native async-profiler initialization to make it portable across operating systems.
• Explain why label state is drained even though OTLP export currently ignores the resulting snapshot.

Testing

./gradlew :agent:test --tests io.pyroscope.javaagent.PyroscopeAgentTest

3 tests completed, 0 failed
BUILD SUCCESSFUL

Closes #344.

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[arnabnandy7]

@korniltsev @korniltsev-grafanista please review this one as well, thanks.