Expose bridge metrics

[jdreichmann]

It would be nice to have the bridge expose some metrics (prometheus style) about it's status, suggestions:

• [x] Matrix users connected to a thirdparty account (as opposed to bridge_puppets_total, this only shows currently active links/connections)-> bridge_connected: Gauge
• [x] Number of bridged rooms (both plumbed and portal) -> bridge_rooms_total: Counter, Labels: {protocol, type: [dm, group]}
• [x] Number of puppets (i.e. matrix users using the bridge, not necessarely connected) -> bridge_puppets_total: Gauge
• [x] Number of remote users (sometimes called ghosts) -> bridge_remote_users_total: Gauge
• [x] total messages/events (labeled by type) bridged / matrix events received-> bridge_messages_total: Counter, Labels: {protocol, type: [text, image, video, file, ...]}
• [x] total messages/events sent to the remote network (aka matrix->$network) -> bridge_matrix_events_total: Counter
• [x] Histograms bridge_matrix_event_bucket and bridge_remote_update_bucket