Monitor meek bridge's usage numbers and alert if it falls below threshold

To make sure that we catch problems like the one in tpo/anti-censorship/pluggable-transports/meek#40001 (closed) early, we should put a system in place that monitors the usage numbers of our meek bridge and alerts us if the numbers fall below a given threshold.

Here's what obfs4proxy's recent public key pinning issue looked like on our meek bridge:

We could regularly fetch the bridge's extrainfo descriptors and extract its usage numbers but there may be a better way to go about this.