Add more metrics to `mntr` command

[JackyWoo]

Description

Right now the only way to introspect RaftKeeper is 4lw command which is based on Zookeeper 3.5. We fould the metrics is too simple to known what is the internal happening.

So we should better enhance the monitoring system. The basic ieade is to enhance 4lw command but not add promethus system, because it is a more simple way and will not introduce stuff for users.

The following are some metrics we need:

1. detailed command statistic: read/write, create/delete/get/multi-write
2. detailed multi-command statistic
3. append entries batch size
4. forwarding batch size
5. more accurate RT time

Are you willing to submit PR?