chore(metrics/prometheus): add files needed for coreth and subnet-evm

[qdm12]

Why this should be merged

So that coreth and subnet-evm can use the metrics package from libevm instead of defining/depending on their own local metrics package.

➡️ coreth PR using this branch
➡️ subnet-evm PR using this branch

Also fixes and refactoring.

How this works

• Add metrics/prometheus/prometheus.libevm.go containing extra (non conflicting) code needed for coreth and subnet-evm
  • Code based on https://github.com/ava-labs/coreth/blob/b6b4dfbc4bfc7f17e4cfe5f3c0fb44944176c884/metrics/prometheus/prometheus.go
  • Function Gatherer renamed to NewGatherer to export gatherer and return a concrete *Gatherer type ("accept interfaces, return concrete types")
  • Fix slice out of range panic in metrics.ResettingTimer switch case
  • Change behavior to return an error if metric type is not supported, instead of ignoring it
  • Define local narrower Registry interface in metrics/prometheus/interfaces.libevm.go
  • Rework switch for metric families to be simpler, notably using ptrTo generic function

How this was tested

CI passing here, in coreth's PR and subnet-evm's PR

[ARR4N]

Why does this need to be in libevm at all? It doesn't use anything from the existing metrics/prometheus package (i.e. it still compiles if moving all of the new files to a different directory). coreth- / subnet-evm-specific functionality must not leak into libevm.

I see that it's still in draft so I may have checked too early, but I also want to save you doing unnecessary work.

[qdm12]

Closed since not needed - coreth and subnet-evm do import libevm, but define their own metrics/prometheus files for their own custom code (even if it's the same for now)