Migrate merge_request_metrics.id to bigint

Timeline

This process needs to be started ASAP - exhausting this column would cause serious harm to the running application.

Process

Following the process outlined here, we must:

• Initialize the conversion (release N)
• Swap the columns (release N + 1)
• Remove the trigger and old integer columns (release N + 2)
• Remove ignore rules (release N + 3)