Resolve "Firing prometheus alerts configured by gitlab are not found by environment"

What does this MR do?

This prevents the bug raised in the issue #204908 (closed) where we could select the incorrect PrometheusAlert (and by association Environment) as a result of parsing a Prometheus alert payload.

This was caused by the existing gitlab_alert_id erroneously providing a PrometheusMetric ID which can be associated to many environments.

We solve this in two ways:

• Add a new field gitlab_prometheus_alert_id that we use to expose the PrometheusAlert id
• If we can't determine which alert to use, we do not associate the PrometheusAlert.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability 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
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Closes #204908 (closed)