Backends by state metric causes 500 error on exposition

The stackdriver_cloudsql_database_cloudsql_googleapis_com_database_postgresql_num_backends_by_state metric appears to have some collection issue when I enabled it. The exposition returns a 500 error.

An error has occurred while serving metrics:\n\n43 error(s) occurred:\n* [from Gatherer #2] collected metric \"stackdriver_cloudsql_database_cloudsql_googleapis_com_database_postgresql_num_backends_by_state\" { label:{name:\"database\" value:\"cloudsqladmin\"} label:{name:\"database_id\" value:\"my-project:mydb\"} label:{name:\"project_id\" value:\"my-project..p_ms:1724089680000} was collected before with the same name and label values\n* [from Gatherer #2] collected metric \"stackdriver_cloudsql_database_cloudsql_googleapis_com_database_uptime\" { label:{name:\"database_id\" value:\"myproject:myotherdb\"} label:{name:\"project_id\" value:\"myproject\"} label:{name:\"region\" value:\"us-central\"} label:{name:\"unit\" value:\"s\"} gauge:{value:60} timestamp_ms:1724089680000} was collected before with the same name and label values\n

Start by reproducing the 500 response with the Cloud SQL backends-by-state metric enabled, then trace how the exporter gathers and exposes the reported duplicate metric samples. Done means the metrics endpoint serves successfully and the affected metrics are not collected more than once with identical names and labels.