Improve section about "System Metrics"

[amotl]

Problem

The tutorials section about how CrateDB integrates with other tools in the system metrics area is currently a bit flat, and also needs to be unflattened.

• https://cratedb.com/docs/guide/integrate/metrics/

Details

Those topics have been identified:

• The content about what the CrateDB Prometheus Adapter can do, is a bit thin. Just referencing it does not add much value other than, well, having the link.
• The page does not even mention the CrateDB JMX/Prometheus Exporter yet.
• Improve Connectivity with Telegraf jmx_exporter#63, and document it.
• Balance properly with the corresponding integrations page.