Add docs and example for OTel JVM runtime metrics

[zeitlinger]

Alternative for #1854

Summary

• Add documentation for using OpenTelemetry's runtime-telemetry module as an alternative to prometheus-metrics-instrumentation-jvm for JVM metrics following OTel semantic conventions
• Add a runnable example project (examples/example-otel-jvm-runtime-metrics) demonstrating combined Prometheus + OTel runtime metrics on a single /metrics endpoint
• Document RuntimeMetricsBuilder configuration options (captureGcCause(), emitExperimentalTelemetry())
• Cover standalone setup, combined setup with PrometheusMetricReader, Java 17 JFR support, and OTel-to-Prometheus metric name mapping

Test plan

• mise run build passes
• mise run lint:super-linter passes
• Manual: java -jar examples/example-otel-jvm-runtime-metrics/target/example-otel-jvm-runtime-metrics.jar then curl localhost:9400/metrics shows both uptime_seconds_total and jvm_* metrics

[zeitlinger]

not related to this PR, but a good opportunity to fix - last time I merged with this check failing and I didn't like it