Document application runtime metrics

[marctc]

Beyla can collect Go and JVM runtime metrics through the shared application_runtime metric feature. JVM collection also provides a configurable sampling interval.

Fields to document

• metrics.features
• discovery.instrument[].metrics.features
• otel_metrics_export.features
• prometheus_export.features
• jvm_runtime_metrics.sampling_interval / OBI_JVM_RUNTIME_METRICS_SAMPLING_INTERVAL

Document application_runtime as an accepted feature value, the runtime coverage it enables, and how the JVM sampling interval affects collection. Include global and per-service examples.