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 ?

[kgeckhart]

Thanks for the PR. I have some concerns regarding the performance of the implementation as reflect deep equals in a double nested loop like this is going to be very expensive.

We have had to do something similar to this in the AWS CloudWatch exporter (https://github.com/kgeckhart/yet-another-cloudwatch-exporter/blob/master/pkg/promutil/migrate.go#L292-L316) but that implementation is much easier because it can hook in to the intermediate type used which has all the labels in a form that can be directly hashed.

The stackdriver exporter doesn't have a clean injection point or a clean representation. The easiest place is the metrics channel but at that point the data is already packaged as a prometheus.Metric where the labels are a []*LabelPair and must be copied to a new map[string]string to fingerprint. I don't have a quick answer ATM just thinking out loud.

Github issues that's likely related, #166