Adds two new metrics for per-slot statistics, network-bytes-in and
network-bytes-out. The network bytes are inclusive of replication bytes
but exclude other types of network traffic such as clusterbus traffic.
#### network-bytes-in
The metric tracks network ingress bytes under per-slot context, by
reverse calculation of `c->argv_len_sum` and `c->argc`, stored under a
newly introduced field `c->net_input_bytes_curr_cmd`.
#### network-bytes-out
The metric tracks network egress bytes under per-slot context, by
hooking onto COB buffer mutations.
#### sample response
Both metrics are reported under the `CLUSTER SLOT-STATS` command.
```
127.0.0.1:6379> cluster slot-stats slotsrange 0 0
1) 1) (integer) 0
2) 1) "key-count"
2) (integer) 0
3) "cpu-usec"
4) (integer) 0
5) "network-bytes-in"
6) (integer) 0
7) "network-bytes-out"
8) (integer) 0
```
---------
Signed-off-by: Kyle Kim <kimkyle@amazon.com>
Signed-off-by: Madelyn Olson <madelyneolson@gmail.com>
Co-authored-by: Madelyn Olson <madelyneolson@gmail.com>
e1d936b339