- Add REQUESTS.current to report the number of requests currently in
   flight, because it's useful for understanding whether we need to
   increase box.cfg.net_msg_max.
 - Add REQUESTS.{rps,total}, because knowing the number of requests
   processed per second can come in handy for performance analysis.
 - Add CONNECTIONS.{rps,total} that show the number of connections
   opened per second and total. Those are not really necessary, but
   without them the output looks kinda lopsided.

Closes #4150

@TarantoolBot document
Title: Document new box.stat.net fields

Here's the list of the new fields:

 - `CONNECTIONS.rps` - number of connections opened per second recently
   (for the last 5 seconds).
 - `CONNECTIONS.total` - total number of connections opened so far.
 - `REQUESTS.current` - number of requests in flight (this is what's
   limited by `box.cfg.net_msg_max`).
 - `REQUESTS.rps` - number of requests processed per second recently
   (for the last 5 seconds).
 - `REQUESTS.total` - total number of requests processed so far.

`CONNECTIONS.rps`, `CONNECTIONS.total`, `REQUESTS.rps`, `REQUESTS.total`
are reset by `box.stat.reset()`.

Example of the new output:
```
---
- SENT:
    total: 5344924
    rps: 840212
  CONNECTIONS:
    current: 60
    rps: 148
    total: 949
  REQUESTS:
    current: 17
    rps: 1936
    total: 12139
  RECEIVED:
    total: 240882
    rps: 38428
...
```