Exclude PrometheusAlerts in count of issues with alerts in service ping

What does this MR do and why?

Changes: This MR cleans up a portion of dead code related to the removal of devopsmonitor Category:Metrics features.

Scope: Any service ping metrics related to PrometheusAlert records. We're going to be dropping the table referenced from Instrumentations::IssuesWithPrometheusAlertEvents, which allows us to simplify the implementation of counts.issues_created_from_alerts. After this merges, the reported value may initially drop, but that's expected and fine.

Context: The Metrics Dashboard (used for monitoring apps deploying using gitlab) was deprecated in 14.7 and removed from the product in 16.0. There are many more files to remove related to this cleanup, but this should be a self-contained chunk.

• grouprespond related issue: Metrics: Delete models & database tables/columns (#397142 - closed)
• Monitor:Metrics removal epic: Monitor: Metrics Deprecation and Removal (&10107)
• Reference MR with all related removals: Draft: Remove all remaining metrics dashboard-r... (!160325 - closed)
• Metrics removal is blocking cells: Drop old unused tables for prometheus alerts (#461838 - closed)

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

How to set up and validate locally

These features were disabled in %16.0 behind the :remove_monitor_metrics feature flag, and have been partially deleted since then. So there's nothing really to test here. It's no longer possible to set up these features.

spec/support/shared_examples/config/metrics/every_metric_definition_shared_examples.rb

@@ -127,8 +127,7 @@ def object_with_schema?(key_path)
     end
 
     let(:ignored_classes) do
-      Gitlab::Usage::Metrics::Instrumentations::IssuesCreatedFromAlertsMetric::ISSUES_FROM_ALERTS_METRICS +
-        Gitlab::Usage::Metrics::Instrumentations::UniqueUsersAllImportsMetric::IMPORTS_METRICS
+      Gitlab::Usage::Metrics::Instrumentations::UniqueUsersAllImportsMetric::IMPORTS_METRICS
     end
 
     def assert_uses_all_nested_classes(parent_module)