Is there a Cadence metric that shows how many activities are waiting to be executed in a task list?

No, there is no a metric exactly like that. There have been several users asking for it but the architecture has made it difficult to support that.

Depends on what you really need, there are different approaches:

• If you want to know if a task list is overloaded so that you can scale up worker fleet or tasklist partition, you should look at schedule_to_start_latency on client metrics, or sync_match_latency on server metrics.
• If you simply want to know if a tasklist is idle or not, you can just look at poll_no_task on client metrics