Geo: Add Gitaly Cluster to geo.staging.gitlab.com

This issue discusses adding Gitaly Cluster to the Geo secondary site on staging (geo.staging.gitlab.com).

We recently demonstrated a working Gitaly Cluster on a Geo deployment in GCP. Adding Gitaly Cluster to the Geo secondary site on staging (geo.staging.gitlab.com) would provide additional assurance that it works, and also allow us to run tests on such a setup regularly (through the daily Geo end-to-end tests currently run on staging. The tests are not specific to Gitaly cluster).

Questions:

• Is this possible? From this writeup the secondary is currently configured on a single node.
• What are the potential downsides? Do they outweigh the potential benefits?