[3pt] detailed iproto requests statistics was implemented

[Totktonada]

Product: Tarantool
Since: 2.10.0-beta2
Root document: https://www.tarantool.io/en/doc/latest/reference/reference_lua/box_stat/
SME: @ locker
Peer reviewer: @

Details

Add new metrics REQUESTS_IN_PROGRESS and REQUESTS_IN_STREAM_QUEUE
to box.stat.net, which contain detailed statistics for iproto requests.
These metrics contains same counters as other metrics in box.stat.net:
current, rps and total.

-- statistics for requests currently being processed in tx thread.
REQUESTS_IN_PROGRESS:
current: -- count of requests currently being processed in the tx thread
rps: -- count of requests processed by the tx thread per second
total: -- total count of requests processed by tx thread

-- statistics for requests placed in queues of streams.
REQUESTS_IN_STREAM_QUEUE:
current: -- count of requests currently waiting in queues of streams
rps: -- count of requests placed in streams queues per second
total: -- total count of requests, which was placed in queues of streams
          for all time

Requested by @EvgenyMekhanik in tarantool/tarantool@b48b333

add to box.stat milestone