Fix missing offset for global aggregations and error ratio metrics

Apply 30s offset to error ratio recordings

This is an oversight from !6209 (merged).

When calculating an error ratio from a source aggregation set of upscaled from self we need to also apply the specified offset. We prepared for that but forgot to add the interpolation to the source string.

Apply 30s offset on global key metrics

The aggregation sets used for services for which we currently evaluate recording rules globally did not have the 30s offset applied. This fixes that.

Contributes to gitlab-com/gl-infra/scalability#2509 (closed)