Create partial index for gitlab-monitor CI metrics

What does this MR do?

Create partial index for gitlab-monitor CI metrics

This creates a partial index intended to speed up queries on ci_builds. Particularly, gitlab-monitor has rather heavy queries. Those have been changed to only look back 7 days and benefit from this index tremendously.

Relates to https://gitlab.com/gitlab-org/gitlab-exporter/merge_requests/101.

Issues:

• https://gitlab.com/gitlab-org/gitlab-exporter/issues/58
• https://gitlab.com/gitlab-com/gl-infra/infrastructure/issues/7724

Conformity

• Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.

Query performance

Actual queries using this index are present in gitlab-monitor. Refer to https://gitlab.com/gitlab-org/gitlab-exporter/merge_requests/101#note_211245325 for plans.