counters: Implement more useful counters.

Right now a lot of errors with MGCP processing are invisible in rate
counters which makes them difficult to trace or even notice in
a production environment. E.g. reaching a limit of MGCP endpoints
is completely invisible even though it's a critical opertion alarm.

Change-Id: I6db68f044255c927dfd534fed880e405ec3ed4d6
3 files changed