Errors when specifying multiple projects and spanner.googleapis.com

Version 0.18.0

When specifying multiple projects

/bin/stackdriver_exporter \
  --google.project-id=gitlab-runway-staging,gitlab-runway-topo-svc-stg \
  --monitoring.metrics-prefixes=spanner.googleapis.com \
  --web.listen-address=:9256 \
  --web.telemetry-path=/metrics

* [from Gatherer #2] collected metric "stackdriver_spanner_instance_spanner_googleapis_com_client_gfe_connectivity_error_count" { label:{name:"client_name"  value:"spanner-go/1.83.0"}  label:{name:"database"  value
:"topology-service"}  label:{name:"directpath_enabled"  value:"false"}  label:{name:"directpath_used"  value:"false"}  label:{name:"instance_config"  value:"unknown"}  label:{name:"instance_id"  value:"topology-ser
vice-dev"}  label:{name:"location"  value:"asia-southeast1"}  label:{name:"method"  value:"Spanner.ExecuteSql"}  label:{name:"project_id"  value:"gitlab-runway-topo-svc-stg"}  label:{name:"status"  value:"Unavailab
le"}  label:{name:"unit"  value:"1"}  gauge:{value:0}  timestamp_ms:1763391420000} was collected before with the same name and label values

All errors are from Gather #2 so it looks like there may be a race condition when you don't specify individual metric prefixes.

I can work around this with:

/bin/stackdriver_exporter \
  --google.project-id=gitlab-runway-staging,gitlab-runway-topo-svc-stg \
  --monitoring.metrics-prefixes=spanner.googleapis.com/instance/cpu \
  --monitoring.metrics-prefixes=spanner.googleapis.com/instance/storage \
  --monitoring.metrics-prefixes=spanner.googleapis.com/instance/sessions \
  --monitoring.metrics-prefixes=spanner.googleapis.com/api/request \
  --monitoring.metrics-prefixes=spanner.googleapis.com/lock_stat \
  --web.listen-address=:9256 \
  --web.telemetry-path=/metrics

And then I don't see the error.

This issue only manifests with multiple projects using the --google.project-id option

Reproduce the issue with the shown stackdriver_exporter command, using multiple --google.project-id values and the broad spanner.googleapis.com prefix. Start by tracing the Gather #2 path and compare it with the individual-prefix workaround. Done means multiple projects can be scraped without duplicate metric collection errors.