Suggests to use a JSON structured log instead

Related to https://gitlab.com/gitlab-org/gitlab-ce/issues/54102

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.

This moves our metric importers into the lib folder. This will allow
them to be autoloaded instead of having to explicitly require them.

This refactors common metrics importer for two reasons. 1. To create a
new pattern that can be followed by future importers that will minimize
dependency collision and 2. To allow EE to more easily extend
dependencies.

Backport of https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/14328

See merge request gitlab-org/gitlab-ce!30003

Remove unused Gitaly feature flags

See merge request gitlab-org/gitlab-ce!30302

When GitLab was migrating to Gitaly, feature flags were used. These are
now out of use, and essentially a no-op. But they do make the output of
chatops ugly and the feature table is selected in full by the
application.