Add Metrics/OpenTelemetry support

It would be nice to have some metrics by default to better diagnose problems and keep track of the bridges.

This is related to https://github.com/mautrix/signal/issues/532 however I think it would be great to have that in every bridge.

Here are some metrics that can likely be exposed by any bridge:

• General system/app metrics, like CPU usage, Memory usage, Go version, threads, file descriptors...
• Amount of users (connected/logged_in/total)
• Amount of puppets
• Amount of messages
• Amount of rooms (private/group)