executor/metrics: distinguish desc a table and explain a SQL on metrics

[jackysp]

What problem does this PR solve?

Issue Number: close #31115

Problem Summary:
SQL like desc/explain does two things,

1. describes the table structure, many frameworks do this automatically,
2. outputs the execution plan of the SQL.
   TiDB needs to distinguish them in metrics.

What is changed and how it works?

Distinguish them by labels.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)

1. start a cluster
2. run some desc table and then some explain SQL
3. check it on Grafana
   

• No code

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

None

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• coocood
• wjhuang2016

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[jackysp]

/run-monitor-test

[sre-bot]

Visit the grafana server at: http://172.16.5.5:31151, it will last for 5 hours

[jackysp]

PTAL @bb7133 @wjhuang2016

[jackysp]

/merge

[ti-chi-bot]

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

Commit hash: 65f778ceb35697d70931eb6b142bdcb38e977728

[jackysp]

/merge

[hawkingrei]

/run-unit-test

[hawkingrei]

Have some fail test in ci. Please fix it.

[hawkingrei]

/hold

[sre-bot]

Code Coverage Details: https://codecov.io/github/pingcap/tidb/commit/7ef001e97cf60e3b702e99b382fee2f2aacc4cc4

[jackysp]

Fixed @hawkingrei

[hawkingrei]

/unhold

[tangenta]

/merge

[ti-chi-bot]

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

Commit hash: ba488b3

[ti-chi-bot]

@jackysp: Your PR was out of date, I have automatically updated it for you.

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/run-all-tests

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