Measure cpu time taken by operations

It would be valuable to measure cpu time consumed by an operation, both as it's running and rolled up by user (to measure resource consumption by a particular user). We could potentially expose this via currentOp, via server status metrics or perhaps in a custom agg pipeline (if by user).

On linux pthread_getcpuclockid() offers a way to access cpu time consumed by a particular thread relatively easily. In turn, we could capture time at opCtx creation, then store the delta on each call to opCtx->checkForInterrupt, then flush the final value into the user level roll up at opCtx destruction.