s1gw_metrics: add histograms for gen_sctp:send/3 delay

https://www.erlang.org/doc/apps/kernel/gen_sctp.html#send/3 clearly
states that gen_sctp:send/3 is blocking, unless non_block_send is set.

With the current architecture of the S1GW, gen_sctp:send/3 may block
the S1GW-MME link (sctp_client) or the eNB-S1GW link (sctp_server)
completely, preventing sending and receiving of other S1AP PDUs.

Ideally, we should rework the S1GW to spawn a separate process for
each received S1AP PDU, so that blocking gen_sctp:send/3 would at
least not block receiving and processing of incoming PDUs.
But this would require quite some effort.

For now, let's add a histogram reflecting min/max/avg/mean/median
delay caused by gen_sctp:send/3, so that we can detect this when
the problem starts biting us.

Change-Id: I7633a6e6182bda3fca4e7562e2b780b2b9f9708f
Related: SYS#7288
---
M config/sys.config
M include/s1gw_metrics.hrl
M src/s1gw_metrics.erl
M src/sctp_common.erl
M src/sctp_proxy.erl
M src/sctp_server.erl
6 files changed, 54 insertions(+), 7 deletions(-)