Replace metrics gathering infrastructure with something more flexible

[Qard]

The current metrics gathering components built on measured lacks some flexibility in a few ways which make it a bit painful for us:

• lack of proper support for async gathering makes some things awkward and hacky
• bulk gathering of multiple metrics from one async source is extra awkward
• little control of what is tracked in the registry globally to apply limits, and therefore little control of overhead
• reporting has too much indirection around it, making it difficult to reason about and control reliably

I think we should start by discussing our needs for metrics gathering and see if we can come up with a high-level description of how the data model and control flow need to fit together, for our use-case.

[axw]

https://github.com/elastic/apm-agent-nodejs/pull/1273 added an option to limit the number of recorded metrics. When the limit is reached, the oldest metric is removed. This is fine (although divergent from the breakdown metrics spec), but we should at least record a log message when this occurs (see the spec again). So we can put this down as one of the needs.