Server statistics over control channel

[valeriansaliou]

Currently, Sonic does not provide any information on its current status and load.

A statistics command (eg. INFO) could be made available via Sonic Channel in control mode, providing information on:

• Server uptime: uptime
• Connected clients: clients_connected
• Total commands: commands_total
• Command latency (best + worst): command_latency_best + command_latency_worst
• Count of open KVs and FSTs: kv_open_count + fst_open_count
• Count of FSTs pending consolidation: fst_consolidate_count

Command flow example:

> INFO
< RESULT uptime(3600) clients_connected(12) commands_total(1002939) command_latency_best(1) command_latency_worst(1) kv_open_count(6) fst_open_count(182) fst_consolidate_count(89)

[valeriansaliou]

Add RocksDB statistics: https://github.com/facebook/rocksdb/wiki/RocksDB-Tuning-Guide#rocksdb-statistics

[valeriansaliou]

1. As a first step, the protocol needs to be defined there (command and expected result).
2. Then, an implementation needs to be done.
3. Finally, the statistics command needs to be added to Sonic Channel libraries.

[valeriansaliou]

As enabling RocksDB statistics adds a 5%-10% overhead, choice has been made not to report those. See: https://github.com/facebook/rocksdb/wiki/Statistics