monitoring: apm-server monitoring metrics maintenance

[carsonip]

Follow up items from #13475

• Determine a long term strategy to maintain apm server monitoring metrics mapping in various places as stated in #13475
• #13638 was created to solve #13475 . See if it is appropriate to be added to the repo and to be maintained.
• Stats mappings are fixed post #13475 but state mappings aren't.
• Stack monitoring uses metrics that are no longer emitted by apm-server: https://github.com/elastic/kibana/pull/187882