Hello Team,
recently we had faced an issue on our production system on 8th Jan 2021 between [16:20-16:40 IST]  wherein one of our mongod[Primary]  instance stop responding version we are using is 4.2.8.
we were able to login to primary instance but  it was getting stuck on 
show dbs/any command   there was no re-index/index building going on which can cause the lock where we checked the logs and we found something 
Flow control is engaged and the sustainer point is not moving. Please check the health of all secondaries
Unable to gather storage statistics for a slow operation due to lock aquire timeout
it remains for almost 10-15 mins in that state post which everything goes healthy.
can you kindly throw some light what could be causing this issue and let us know anything required from our side.