If you set Mongo ruby driver log level to "debug" in a Rails console, you'll see the following logs while idling.
I think no log is needed for toggling between "primary" vs. "primary" [awaited]
MONGODB | Server description for tablecheck-staging-shard-00-00.uhz3j.mongodb.net:27017 changed from 'primary' to 'primary'. MONGODB | There was a change in the members of the 'ReplicaSetWithPrimary' topology. MONGODB | Server description for tablecheck-staging-shard-00-00.uhz3j.mongodb.net:27017 changed from 'primary' to 'primary' [awaited]. MONGODB | There was a change in the members of the 'ReplicaSetWithPrimary' topology. MONGODB | Server description for tablecheck-staging-shard-00-00.uhz3j.mongodb.net:27017 changed from 'primary' to 'primary'. MONGODB | There was a change in the members of the 'ReplicaSetWithPrimary' topology. MONGODB | Server description for tablecheck-staging-shard-00-00.uhz3j.mongodb.net:27017 changed from 'primary' to 'primary' [awaited]. MONGODB | There was a change in the members of the 'ReplicaSetWithPrimary' topology.