In 3.0 we no longer have db locks, but we have some other metrics we can use to estimate lock contention:
Percentage of lock acquisitions that had to wait
serverStatus.locks.Collection.acquireWaitCount.{R,W} / serverStatus.locks.Collection.acquireCount.{R,W}
Average acquire time per acquisition that waited
serverStatus.locks.Collection.timeAcquiringMicros.{R,W} / serverStatus.locks.Collection.acquireWaitCount.{R,W}

They should be exposed in mongostat.