iproto: implement getting statistics for iproto streams

Currently there are several statistic metrics for iproto, which can
be obtained by using `box.stat.net`: `CONNECTIONS`, `REQUESTS`, `SENT`
and `RECEIVED`. Add new metric `STREAMS` which contain statistics for
iproto streams.

Part of #6293

@TarantoolBot document
Title: getting statistics for iproto streams was implemented
Add new  metric `STREAMS` to `box.stat.net`, which contain statistics
for iproto streams. `STREAMS` contains same counters as `CONNECTIONS`
metric in `box.stat.net`: current, rps and total. For example:
STREAMS:
  current: 0
  rps: 17980
  totat: 112623