infoschema: add 3 fields to statement summary table (#14096)

[djshow832]

conflicting files:
const.go

What problem does this PR solve?

Add 3 more fields to events_statements_summary_by_digest:

• SUMMARY_END_TIME: the end time of current summary.
• SUM_BACKOFF_TIMES: the total backoff times of this kind of SQL.
• PREV_SAMPLE_TEXT: the previous SQL before current SQL if current SQL is commit.

What is changed and how it works?

• SUMMARY_END_TIME: the end time will change if refreshInterval changes.
• PREV_SAMPLE_TEXT: the identifier of each kind of SQL is composed of previous digest and current digest, so it can distinguish different types of transactions.

Check List

Tests

• Unit test

Code changes

• Has exported variable/fields change

Side effects

• Possible performance regression
• Increased code complexity

Related changes

• Need to cherry-pick to the release branch
• Need to update the documentation

Release note

• Add 3 more fields to table events_statements_summary_by_digest:
  SUMMARY_END_TIME: the end time of current summary.
  SUM_BACKOFF_TIMES: the total backoff times of this kind of SQL.
  PREV_SAMPLE_TEXT: the previous SQL before current SQL if current SQL is commit.

[djshow832]

/run-all-tests

[bb7133]

LGTM

[lonng]

LGTM