[vtgate] Backport vitessio/vitess#11167

[brirams]

Motivation

As the title suggests, this is a backport of a change that was made in upstream change for the v15 release.

Changes

Most all changes there were applied to our v12 branch except for:

• changes to the endtoend flags tests(they don't exist in v12)
• changes to the v15 changelog(obviously doesn't exist in v12)

Testing

• new benchmark tests added
• some unit tests pass(most end up being marked as flaky in future release)
• deployed this to dev and started up as expected with the new flag off and on

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