Issue title: Channel full metric needs to be updated
 Issue body: ## Bug Report

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### What version of TiKV are you using?
\>= v3.0.0

### What operating system and CPU are you using?
doesn't matter

### Steps to reproduce
Just see the channel full metric

### What did you expect?
The channel full metric should be updated when some channels are full.

### What did happened?
The channel full metric is always empty even when some peer's channel are full.

![image](https://user-images.githubusercontent.com/21253569/122333307-85034880-cf6a-11eb-8f36-5a7ace670efb.png)
I find the `tikv_channel_full` metric has been removed for a long time since v3.0.0. It should be updated.
 Comments: 
 Comment 0: This metrics has been removed by https://github.com/tikv/tikv/pull/4232.
 Comment 1: /severity Minor
 Comment 2: /severity Moderate
 Comment 3: /remove-severity
 Comment 4: /remove-severity minor
 Comment 5: @BusyJay PTAL, thanks.
{'source': 'GithubIssues', 'repo_name_with_owner': 'tikv/tikv', 'issue_id': 923434581, 'repo_stars': 8908.0}