Group all metrics-related configuration options in the metrics section

We have a bunch of config options related to metrics that end up in different categories:

[Core configuration options](https://www.elastic.co/guide/en/apm/agent/java/current/config-core.html)
- [`breakdown_metrics`](https://www.elastic.co/guide/en/apm/agent/java/current/config-core.html#config-breakdown-metrics)

[JMX configuration options](https://www.elastic.co/guide/en/apm/agent/java/current/config-jmx.html)
- [capture_jmx_metrics](https://www.elastic.co/guide/en/apm/agent/java/current/config-jmx.html#config-capture-jmx-metrics)

[Metrics configuration options](https://www.elastic.co/guide/en/apm/agent/java/current/config-metrics.html)
- [dedot_custom_metrics](https://www.elastic.co/guide/en/apm/agent/java/current/config-metrics.html#config-dedot-custom-metrics)
- [metric_set_limit](https://www.elastic.co/guide/en/apm/agent/java/current/config-metrics.html#config-metric-set-limit)

[Reporter configuration options](https://www.elastic.co/guide/en/apm/agent/java/current/config-reporter.html)
- [metrics_interval](https://www.elastic.co/guide/en/apm/agent/java/current/config-reporter.html#config-metrics-interval)
- [disable_metrics](https://www.elastic.co/guide/en/apm/agent/java/current/config-reporter.html#config-disable-metrics)

Can we merge all of these into the [Metrics configuration options](https://www.elastic.co/guide/en/apm/agent/java/current/config-metrics.html) section?
We'll need to be careful and update references to these options, if there are any. The docs build should catch dead links.