Add metrics to retry_stuck_testcases cronjob

[google-labs-jules[bot]]

This change adds metrics to the retry_stuck_testcases cronjob to provide better visibility into its operation. The new metrics track the number of testcase candidates and the various outcomes of the process, such as successful restarts and different reasons for skipping. These metrics are also collected during --dry-run executions to allow for safe monitoring and analysis.

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PR created automatically by Jules for task 17630666630767645746

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[hunsche]

/gcbrun

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