Add gitlab_main_cell schema to PrometheusAlertIndicator

What does this MR do and why?

This updates PrometheusAlertIndicator to support the gitlab_main_cell schema.

Script to include gitlab_main_cell schema settings in application_settings:

Note that for now since gitlab_main_cell schema exists on gitlab_main database (patroni), so gitlab_main_cell configuration is a copy of gitlab_main schema configuration.

application_setting = ApplicationSetting.last
prometheus_alert_db_indicators_settings = application_setting.prometheus_alert_db_indicators_settings

application_setting.update(prometheus_alert_db_indicators_settings: prometheus_alert_db_indicators_settings.deep_merge({
  "apdex_slo" => {
    "main_cell" => 0.997
  },
  "wal_rate_slo" => {
    "main_cell" => 70000000
  },
  "apdex_sli_query" => {
    "main_cell" => "avg_over_time(gitlab_service_apdex:ratio_5m{env='gstg',environment='gstg',monitor='global',type='patroni'}[5m])"
  },
  "wal_rate_sli_query" => {
    "main_cell" => "avg(postgres:pg_xlog_bytes_per_second:rate5m{env='gstg', type='patroni'})"
  }
}))

Related to: #433196 (closed)

Change Requests (CR) issues

• gstg - gitlab-com/gl-infra/production#17207 (closed)
• gprd - gitlab-com/gl-infra/production#17208 (closed)

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