mgcp_ratectr: add stats items to monitor trunk usage

We are currently counting events in rate counters, but there is
currently no way to get a sample of the current situation of the trunk
usage. In particular how many endpoints are currently in use.

Change-Id: Ib7b654168dc3512f55e45cc4755dc1f6f423d023
Related: SYS#5535
6 files changed