Add warnings to the slow log and the slow query table

[time-and-fate]

Enhancement

As the title says, we need to add warnings to the slow log and the information_schame.slow_query system table. This will help to analyze some problems.

[time-and-fate]

Implementation

• Add a field in StatementContext to store the "extra warnings".
  • If a warning is expected to be output only under some conditions (like in EXPLAIN or EXPLAIN VERBOSE) but it's not under such conditions now, it is considered as an extra warning.
• Add a "Warnings" field after the "Result_rows" field in the slow log.
  • The content of this field is all normal warnings and the new "extra warnings".
  • The format of this field is JSON, marshalled from the following type definition:

[]struct {
   Level   string
   Message string
   IsExtra bool `json:",omitempty"`
}

• Add a "Warnings" column after the "Result_rows" column in the information_schema.slow_query table.
  • The content of this column is the same as the "Warnings" field in the slow log.

Tests

• Normal warnings are correctly printed in the slow log and the slow query table.
• Extra warnings are correctly printed in the slow log and the slow query table.

https://github.com/pingcap/tidb/pull/39884/files#diff-c6ebd628dc7de8f85e40966ac18d01e36f12da0899619f75bbc81895181b150c

https://github.com/pingcap/tidb/pull/39884/files#diff-0d6dca36cb3b240cb07fe78517dcf02e1c58c207861edf73cf0e2616b77e8b93