enb_proxy/mme_registry: add per-MME counters for connection events
Register a set of per-MME counters when an MME is added to the pool,
and increment them from enb_proxy at the relevant state transitions:
- selected (MME chosen for a connection attempt)
- conn_est_timeout (SCTP connection establishment timed out)
- conn_est_failure (SCTP connection establishment failed)
- s1setup_rsp (S1 SETUP RESPONSE received successfully)
- s1setup_failure (S1 SETUP FAILURE received from MME)
- s1setup_rsp_timeout (timed out waiting for S1 SETUP RESPONSE)
A new global aggregate counter ?S1GW_CTR_ENB_PROXY_MME_SELECTED is
also added alongside the existing ?S1GW_CTR_ENB_PROXY_MME_SELECT_ERROR.
Change-Id: Ie0149c1ad0754af6d8f5f95d4e8919993eac3760
Related: SYS#7052
