Replace metrics gathering infrastructure with something more flexible

The current metrics gathering components built on [measured](https://npmjs.org/package/measured-core) 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.