Basically enables us to use `test.WithAllDatabases` when testing
internal HTTP APIs, as this would otherwise result in Prometheus
complaining about already registered metric names.
* Generic-based internal HTTP API (tested out on a few endpoints in the federation API)
* Add `PerformInvite`
* More tweaks
* Fix metric name
* Fix LookupStateIDs
* Lots of changes to clients
* Some serverside stuff
* Some error handling
* Use paths as metric names
* Revert "Use paths as metric names"
This reverts commit a9323a6a34.
* Namespace metric names
* Remove duplicate entry
* Remove another duplicate entry
* Tweak error handling
* Some more tweaks
* Update error behaviour
* Some more error tweaking
* Fix API path for `PerformDeleteKeys`
* Fix another path
* Tweak federation client proxying
* Fix another path
* Don't return typed nils
* Some more tweaks, not that it makes any difference
* Tweak federation client proxying
* Maybe fix the key backup test
