Tracing: Deprecate usage ping & snowplow metrics

Why are we doing this work

In 15.0, we disabled the Tracing features via a feature flag. Before removing the related code, we need to ensure that service ping and any tracing-related metrics are able to be safely removed.

⚠ Removal should take place in %15.2 or later.

Tracking issue for deprecation & removal: Monitor: Tracing Deprecation (#346540 - closed)

Relevant links

• Snowplow metrics removal process: slack thread | screenshot of discussion
• Usage ping metrics removal process: https://docs.gitlab.com/ee/development/service_ping/metrics_lifecycle.html#remove-a-metric
• Usage ping fallbacks: https://docs.gitlab.com/ee/development/service_ping/implement.html#fallback-values-for-service-ping

Implementation plan

Follow steps outlined in linked documentation for the following metrics removals:

• project:operations:tracing.external_url_populated (snowplow) | app/controllers/projects/settings/operations_controller.rb#L72
• projects_with_tracing_enabled (usage ping) | lib/gitlab/usage_data.rb