[Improvement] TMS update-stats: remove custom- prefix from built-in metrics names

### What would you like to be improved?

Metrics produced by built-in `update table stats and metrics` job currently use `custom-*` naming (for example `custom-data-file-mse`). This causes friction in monitor/evaluator rule configuration and cross-environment consistency, because users need to discover and map metric names before rule setup.

### How should we improve?

- Standardize built-in metrics naming by removing the `custom-` prefix.
- Keep backward compatibility (migration or alias behavior) so existing monitor rules do not break unexpectedly.
- Update related docs/examples (`submit-update-stats-job`, `monitor-metrics`, and troubleshooting) with the new naming convention.
- Add tests covering produced metric names and monitor rule evaluation with the new names.

Parent Epic: #10360

Start by locating the built-in “update table stats and metrics” job and the code that produces `custom-*` metric names. Then review the `submit-update-stats-job`, `monitor-metrics`, and troubleshooting documentation, along with existing monitor-rule tests; done means standardized names, backward compatibility, updated examples, and coverage for metric production and rule evaluation.

Written by the indexing model from the issue text.