public long done() {
    if (enabled) {
      if (!done) {
        lastCheckpointNanos = System.nanoTime();
        done = true;
      }
      return lastCheckpointNanos - startNanos;
    }
    return -1;
  }
public long done ( ) { if ( enabled ) { if ( ! done ) { lastCheckpointNanos = System . nanoTime ( ) ; done = true ; } return lastCheckpointNanos - startNanos ; } return - 1 ; }
Indicate we are done (stop the timer). Once you call this
method, subsequent calls have no effect.

@return time in nanoseconds from the start of this metric, or -1
if {@code false} was passed in the constructor