[2pt] getting network statistic per thread was implemented

[Totktonada]

Product: Tarantool
Root document: https://www.tarantool.io/en/doc/latest/reference/reference_lua/box_stat/
SME: @ EvgenyMekhanik
Peer reviewer: @

Details

User has possibility to run several iproto threads, but at the moment
he can get only general statistics, which combines statistics for all
threads. Now box.stat.net.thread has been implemented to get the same
statistics as when using box.stat.net, but for a thread. User can call
box.stat.net.thread() as a function to get general network statistics
per threads. For example for two iproto threads:

---
- - SENT:
      total: 0
      rps: 0
    CONNECTIONS:
      current: 0
      rps: 0
      total: 0
    REQUESTS:
      current: 0
      rps: 0
      total: 0
    RECEIVED:
      total: 0
      rps: 0
  - SENT:
      total: 0
      rps: 0
    CONNECTIONS:
      current: 0
      rps: 0
      total: 0
    REQUESTS:
      current: 0
      rps: 0
      total: 0
    RECEIVED:
      total: 0
      rps: 0
...

Also user can indexed it as a table by thread number. For example
box.stat.net.thread[1] returns network statistics for first iproto
thread:

---
- SENT:
    total: 0
    rps: 0
  CONNECTIONS:
    current: 0
    rps: 0
    total: 0
  REQUESTS:
    current: 0
    rps: 0
    total: 0
  RECEIVED:
    total: 0
    rps: 0
...

Requested by @EvgenyMekhanik in tarantool/tarantool@6fd94e2