bindinfo: add status vars for 'last_plan_binding_update_time'

[Reminiscent]

What problem does this PR solve?

Issue Number: close issue#26199

Problem Summary:
Add status vars for 'last_plan_binding_update_time'. We can use show status like 'last_plan_binding_update_time'; to see the last plan binding update time in the server.

What is changed and how it works?

Add status variable to record the last plan binding update time.

Check List

Tests

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

• Details of the last plan binding update time are now available in the last_plan_binding_update_time status variables

[Reminiscent]

/cc @eurekaka

[Reminiscent]

/cc @morgo

[morgo]

Please fix the linter issues, but otherwise this is looking good:

[2021-07-19T06:59:11.974Z] linting

[2021-07-19T06:59:12.232Z]   ✘  https://revive.run/r#var-naming  don't use underscores in Go names; var update_time should be updateTime  

[2021-07-19T06:59:12.232Z]   bindinfo/bind_test.go:2130:2

[2021-07-19T06:59:12.232Z] 

[2021-07-19T06:59:12.232Z]   ✘  https://revive.run/r#var-naming  don't use underscores in Go names; var update_time1 should be updateTime1  

[2021-07-19T06:59:12.232Z]   bindinfo/bind_test.go:2132:2

[2021-07-19T06:59:12.232Z] 

[2021-07-19T06:59:12.232Z]   ✘  https://revive.run/r#var-naming  don't use underscores in Go names; var update_time2 should be updateTime2  

[2021-07-19T06:59:12.233Z]   bindinfo/bind_test.go:2135:2

[2021-07-19T06:59:12.233Z] 

[2021-07-19T06:59:12.233Z] ✘ 3 problems (3 errors, 0 warnings)

[2021-07-19T06:59:12.233Z] 

[2021-07-19T06:59:12.233Z] Errors:

[2021-07-19T06:59:12.233Z]   3  var-naming  

[Reminiscent]

@morgo Update, PTAL. Besides, @eurekaka thinks that the name stat.go will be ambiguous with the naming of statistics in the optimizer. What do you think?

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[REVIEW NOTIFICATION]

This pull request has been approved by:

• eurekaka
• rebelice

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[Reminiscent]

@eurekaka Update. PTAL

[eurekaka]

/merge

[ti-chi-bot]

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

Commit hash: b1fa991