datastore-prometheus-metrics flag not implemented for Spanner

[srlightbody]

What platforms are affected?

linux

What architectures are affected?

amd64

What SpiceDB version are you using?

1.37.0, also tested v1.33.1-hotfix-enterprise.v1.hotfix.v1

Steps to Reproduce

Configure a spanner datastore with SpiceDB
Run spicedb with --datastore-prometheus-metrics set to false and Prometheus configured
Curl the configured metrics path that is serving SpiceDB metrics
All spanner metrics will still be visible, spicedb_cloud_google_com_go_spanner_gfe_latency_count for example.

Expected Result

All spanner datastore metrics are not exposed on SpiceDBs prometheus endpoint when the datastore-prometheus-metrics flag is set to false.

Actual Result

All spanner datastore metrics continue to be exposed on SpiceDBs prometheus endpoint when the datastore-prometheus-metrics flag is set to false.