Unify redundant requests to GCE Disks

[7onn]

Describe the problem/challenge you have
We currently have three collectors making API calls to retrieve disk-snapshot-related information.

• gce_disk_snapshot_age_days
• gce_disk_snapshot_amount
• gce_is_old_snapshot

As they run asynchronously, each one of them is instantiating their own GCP client and Compute Service.

Describe the solution you'd like
For performance purposes, I'd like to take advantage of a single instantiation of these mentioned agents to provide these metrics. Plus removing the gce_is_old_snapshot as this information is now available through the snapshot age.

Anything else you would like to add:
n/a

Environment:
n/a