Prometheus stuff needs refactoring

*This issue was originally created by [**@neilalexander**](https://github.com/neilalexander) at .*

We use `init()` a lot to register prometheus collectors which is bad for WASM, and dupe registrations caused problems for the server signing key tests too.

Start by locating the init() functions that register Prometheus collectors and inspect how those registrations are used by the server signing key tests. Determine a registration approach that works under WASM and prevents duplicate collectors. Done means the refactoring is complete without duplicate-registration failures and the affected tests still pass.

Written by the indexing model from the issue text.