Add metric to record the traffic for local cache

[walleliu1016]

Feature request:

The traffic for scheduler include two parts:

1. traffic between peers
2. traffic back to source

absence the traffic response with local cache.

Use case:

1. add localcache traffic metric if response with localcache
2. add back-to-source traffic metric if response with downstream
3. add p2p traffic metric if response with peers

UI Example:

[divaspathak]

Hello @walleliu1016 and @gaius-qi, I want to work on this issue. Can you please assign it to me?

[walleliu1016]

Hello @walleliu1016 and @gaius-qi, I want to work on this issue. Can you please assign it to me?

I have not finished yet, as i am not the committer in dragonfly. if you interested in this issue, you can ask @gaius-qi assign it to you. wish your pr.