Add flag --skip-metadata-lock-check

[meiji163]

Description

Since #1536, performance_schema.metadata_locks is required to check the rename session holds the metadata lock on the migrated table during cut-over. On some setups such as Aurora RDS performance_schema is not enabled by default and it may be infeasible to enable.
This PR adds --skip-metadata-lock-check flag to skip the check in this case.

Although the case described in #1536 is uncommon, data integrity is top priority for gh-ost, so we recommend to enable performance_schema if possible.

Closes #1615

In case this PR introduced Go code changes:

• contributed code is using same conventions as original code
• script/cibuild returns with no formatting errors, build errors or unit test errors.

[Copilot]

Pull request overview

This pull request adds a new command-line flag --skip-metadata-lock-checks to allow users to bypass metadata lock validation during cut-over when performance_schema.metadata_locks cannot be enabled (e.g., on Aurora RDS). This addresses environments where enabling performance_schema may be infeasible while acknowledging a small risk of data loss.

Changes:

• Added --skip-metadata-lock-checks flag to bypass metadata lock checks at cut-over
• Enhanced StateMetadataLockInstrument() to handle cases where performance_schema is disabled
• Added validation logic in initiateApplier() to conditionally bail out or warn based on flag

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
go/cmd/gh-ost/main.go	Added --skip-metadata-lock-checks flag definition and updated description for --allow-setup-metadata-lock-instruments flag
go/base/context.go	Added SkipMetadataLockChecks boolean field to MigrationContext
go/logic/applier.go	Modified StateMetadataLockInstrument() to handle ErrNoRows when performance_schema is disabled
go/logic/migrator.go	Added conditional logic to check SkipMetadataLockChecks flag and bail out or warn accordingly
doc/command-line-flags.md	Added documentation for both skip-metadata-lock-check and allow-setup-metadata-lock-instruments flags

Comments suppressed due to low confidence (1)

doc/command-line-flags.md:257

• Inconsistent flag name in documentation text. The flag is defined as --skip-metadata-lock-checks (plural) but the documentation refers to --skip-metadata-lock-check (singular). Update to use --skip-metadata-lock-checks.

By default `gh-ost` performs a check before the cut-over to ensure the rename session holds the exclusive metadata lock on the table. In case `performance_schema.metadata_locks` cannot be enabled on your setup, this check can be skipped with `--skip-metadata-lock-check`. 

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[saimitta-looker]

@meiji163 Can we include this v1.1.18? v1.1.18-rc introduce metadata checks which require access to performance_schema and option to skip this check should be included in the same release candidate as well to avoid breaking users who can't turn on performance_schema metadata_locks