With the recent updates to Firebase, there were two new usage metrics introduced to Cloud Firestore:
Active connections
Snapshot listeners
About active connections, I found out that there is a limit of 1000000 (1M) concurrent connections. So I can understand why that metric would be useful, but what about the snapshot listeners metrics? That number looks quite scary and Stackdriver Monitoring does not mention any cost or limit for "The number of snapshot listeners across all active connections".
A listener does not cost anything extra than the document reads it normally incurs. Please refer to the documentation on billing to better understand how that works.
Please contact Firebase support directly with additional billing concerns.