metrics: update the mvcc db metrics of etcd

[nolouch]

What problem does this PR solve?

Issue Number: Close #4399

What is changed and how does it work?

metrics: update the mvcc db metrics of etcd
- distinguish between physical and logical

Check List

Tests

• Unit test
• Integration test
  

Release note

None.

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• rleungx

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[nolouch]

/merge

[ti-chi-bot]

@nolouch: It seems you want to merge this PR, I will help you trigger all the tests:

/run-all-tests

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[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 5edac19