Make BridgeDB report internal metrics

We're done with legacy/trac#9316 (moved), which means that we have code in place that allows BridgeDB to export metrics. So far, all metrics are user-centric, meaning that they focus on how BridgeDB users interact with the system. BridgeDB-centric metrics would help us debug and understand BridgeDB. The following come to mind:

• Number of bridges per distribution ring.
• Number of bridges per transport, similar to assignments.log (originally proposed in legacy/trac#14453 (moved))
• Number of requests for which we had no bridges.

We could also incorporate bridge assignments in our metrics, so we don't have to report them separately in the assignments.log file (see legacy/trac#29480 (moved)). Let's not forget to update BridgeDB's metrics specification.

[In the context of #31274 (closed) O2.1 - Create an evaluation framework and collect data to better monitor and evaluate current bridge selection and distribution processes.]