I understand the columns last_sent_tim, last_received_time much better now. Thanks for the replies. But I still can't understand that there is nothing to monitor the log sent information (in terms of time) on Primary directly.
Also to your question
"If you have multiple secondary then who told that the last_sent_time is the same to all these replicas?!? "
Since we have one row per secondary, this should not be a problem right? it can show the send times per secondary.
My main requirement is that I want to monitor if there is any* progress at from the Primary in terms of sending logs to secondary. I can't use the queue size on primary as this can increase or decrease.
In Database Mirroring we have the counter 'Log Bytes Sent/sec' which is not null on Principal and it updates (cumulative) every time a log is sent to Mirror. I do not see any such counter for alwayson.