OsmoMGW has a lot of nice built-in statistics (rate_ctr,...) but it
seems the only way to look at them is via the VTY. While libosmocore
contains automatic exposure of all rate counters via CTRL, the CTRL
interface simply is not used by osmo-mgw so far.
Closes: OS#4441
Change-Id: I7ed6bdb9f4749c24ca11a5905a620546cfe42952
