Some Questions About `log_send_rate`, `log_send_queue_size`, `redo_queue_size`, and `redo_rate`.

Hello,  I've recently been trying to monitor the latency of the Available Group. 

Regarding logsend latency and redo latency, I hope to monitor them using the `log_send_queue_size` / `log_send_rate` and `redo_queue_size` / `redo_rate` metrics in the `dm_hadr_database_replica_states` DMV.

However, in the process, I noticed that even in busy systems, `log_send_queue_size` and `log_send_rate` are often 0, whereas in idle systems, `redo_rate` is never 0.

 Could you please explain the specific definitions of `log_send_rate` and `redo_rate`? Why is `redo_rate` not zero when no data synchronization is taking place? In a system where data synchronization is occurring, `log_send_rate` and `log_send_queue_size` may be zero. My understanding is that log sends occur very quickly, while the monitoring granularity is not fine-grained enough—is that correct?

Hope someone from the community comment on this.