Add a flag, perhaps "maxTimeMS", to the isMaster command that initiates long-polling. The server will not respond until maxTimeMS expires or there is a server state change that might interest the client, such as a replica set member role change or a replica set config change.

Also add a field, perhaps "awaitedTimeMillis", to the reply, which is approximately the time the server waited between receiving the requested and assembling the reply. The client can subtract awaitedTimeMillis from the client's round trip latency measurement to estimate the network latency.