Instrument "429 Too Many Requests" responses from GCS

Context

This is a followup from gitlab-com/gl-infra/delivery#2161 (comment 774612707).

Problem

Right now we're missing a metric to track the number of 429 Too Many Requests responses from a storage backend, such as GCS. This means that we have no idea of how many requests are currently rejected due to rate limits.

Solution

Create a new Prometheus counter to keep track of 429 Too Many Requests responses from the storage backend. The immediate use case is GCS. For GitLab.com, we should then surface this metric in the Storage Detail Grafana dashboard.