For both mongos and mongod, it would be helpful to have an indicator of how long server discovery is taking.

For example, if a replica set has an election, understanding the time it took a mongos to discover the new primary is very useful to diagnose issues, by logging "It took X milliseconds to find a valid primary" This should measure the time from when a active primary is not detected until a new primary is discovered.

Ideally, a single log line reports

• the total time, as .attr.durationMillis, that any server needed to determine the state of another server
• the desired state that needs to be determined or the reason the state of the other server needed to be known

Acceptance criteria:

server description record in topology change message includes the amount of time spent in previous topology state.