add MDCX command statistics to osmo-mgw

Add a counter group for MDCX commands. The group contains counters for
successful connection processing as well as various error conditions.
This provides a quick overview of MDCX failures on each trunk throughout
the lifetime of the osmo-mgw process.

The counters are displayed by 'show mgcp stats' and 'show rate-counters'.

Change-Id: I79c27425ba40c3a85edc6cd846cba325d847298c
Depends: Ia2004f8063f3a50b5d7a838ebe8a784a47fcc50d
Related: OS#2660
3 files changed