Setup PrometheusAlertIndicator support for gitlab_sec schema

What

See #481330 (comment 2182102746) discovery of Sentry error https://new-sentry.gitlab.net/organizations/gitlab/issues/986572/

Mirroring Setup db indicator to gitlab_main_cell schema (#433196 - closed), we need to update PrometheusAlertIndicator and application settings to include gitlab_sec schemas in healthcheck for proper reporting of throttling SLIs against BBMs.

Proposal

1. Update mapping for alert indicators to include gitlab_sec to avoid key error
2. Update prometheus_alert_db_indicators_settings to include gitlab_sec reference