feat: [metrics] Create metrics framework and add connection metrics

[charlesdong1991]

Purpose

Linked issue: close #390

Brief change log

This PR adds a metrics framework and connection level metrics.

NOTE:
There is an parity from Java, which is the scopeguard for in-flight on cancellation, because Java callback model doesn't have equivalent to tokio future cancellation. And if a tokio future is dropped mid-execution, it would skip decrement and cause gauge drift. So i think it's important to add "scopeguard" to avoid that.

And I can have writer and scanner metrics as follow-up PRs.

Tests

All tests are passed locally.

[fresh-borzoni]

@charlesdong1991 Ty for the PR, left some comments
PTAL

[charlesdong1991]

Thanks for reviews, i made some changes and leave a question. PTAL @fresh-borzoni

[fresh-borzoni]

@charlesdong1991 LGTM overall, small comment about misleading doc, and we can do simple math to match Java here, at least semantically. PTAL

[charlesdong1991]

and we can do simple math to match Java here, at least semantically. PTAL

btw, just to clarify before making changes, do you mean to update code to fix header overhead? or to update the doc to reflect this? @fresh-borzoni

[fresh-borzoni]

and we can do simple math to match Java here, at least semantically. PTAL

btw, just to clarify before making changes, do you mean to update code to fix header overhead? or to update the doc to reflect this? @fresh-borzoni

Sorry, I should have been more specific - I meant code @charlesdong1991

[charlesdong1991]

Gotcha, thanks @fresh-borzoni will do a quick change, should be straightforward 👍

[charlesdong1991]

i updated code to calculate total size "manually", PTAL @fresh-borzoni

thanks again!

[fresh-borzoni]

@charlesdong1991 TY, LGTM