Hello Jai,
According to the doc [1], Custom metrics uses the same elements that the built-in Cloud Monitoring metrics use.
Doing further research on your issue, I couldn’t find reference specific to your query. However, you may find this documents [2],[3] helpful to get a better understanding.
[1] https://cloud.google.com/monitoring/custom-metrics/creating-metrics
[2] https://cloud.google.com/logging/docs/logs-based-metrics
[3] https://cloud.google.com/logging/quotas#metric-limits