Ensure ID is big enough for prometheus_metrics records

What does this MR do?

This is because https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9959/diffs#0e13c38c6cf3590a48f24587785e0c00d393fb96 adds a new default Prometheus metric, and the db/migrate/20180831164910_import_common_metrics.rb migration is run before ee/spec/migrations/allow_prometheus_alerts_per_environment_spec.rb so there's already 21 records in the prometheus_metrics table => 💥

Does this MR meet the acceptance criteria?

Closes #11054 (closed).

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
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• Style guides
• Database guides
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Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

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• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
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