Record feature category metrics for SLIs that have feature categories on their source labels

This adds the possiblity to set fromSourceMetrics as a featureCategory on the SLI.

Doing so will cause gitlab:component:feature_category: type recording rules to be generated with the feature_category as an aggregation label.

This also adds this for the rails_requests component, so we have a way to feed these metrics into the error budget.

This is required for gitlab-com/gl-infra/scalability#1229 but blocking this on !4038 (closed) because we want to avoid feeding these into the stage group metrics automatically.