This documents the various knowledge needed to add new GitLab
self-monitoring Prometheus metrics.

In order to avoid confusion between Gitlab::Importers module and other
like named modules, this renamed it to Gitlab::DatabaseImporters.

Remove CommonMetricsImporter patch and change all references to
CommonMetrics::Importer. Move specs into their appropriate folders. Also
cleans up some common_metric importer namespacing.

In order to avoid confusion between Gitlab::Importers module and other
like named modules, this renamed it to Gitlab::DatabaseImporters.

Remove CommonMetricsImporter patch and change all references to
CommonMetrics::Importer. Move specs into their appropriate folders. Also
cleans up some common_metric importer namespacing.

Extending from ActiveRecord::Migration is deprecated, but was still used
in a bunch of places.

Extending from ActiveRecord::Migration is deprecated, but was still used
in a bunch of places.