Fix "metric was collected before" error

[colinleroy]

Hello,

Since one or two weeks, our stackdriver_exporter errors out with logs similar to:

collected metric "stackdriver_network_security_policy_networksecurity_googleapis_com_https_request_count" { label:{name:"backend_target_name"  value:"****-https-lb-spectrum-back"}  label:{name:"blocked"  value:"true"}  label:{name:"location"  value:"global"}  label:{name:"policy_name"  value:"preconfigured-rules-policy"}  label:{name:"project_id"  value:"****-production"}  label:{name:"unit"  value:"1"}  gauge:{value:0}  timestamp_ms:1746012857310} was collected before with the same name and label values

After investigation, it comes from the fact that some of these metrics are returned twice by Stackdriver.

This PR adds a filtering to reportTimeSeriesMetrics() in order to avoid sending duplicate time series to the Prometheus Collector.

I hope this helps and I hope it's good enough, I'm not very well-versed in Go.

Thanks!

[aarozhkov]

@colinleroy
Stack on this too.
Try to use binary builded from your PR.

Lost other valid metrics.
Example stackdriver_pubsub_subscription_pubsub_googleapis_com_subscription_ack_latencies_bucket
I have a lot of subscribtion_ids.
Left with only on subscription metric.

[colinleroy]

Were they metrics with at least one unique key/value label ?