code: private boolean triggerStackTraceSampleInternal(final ExecutionJobVertex vertex) {
  		assert(Thread.holdsLock(lock));
  
  		if (shutDown) {
  			return false;
  		}
  
  		if (!pendingStats.contains(vertex) &&
  			!vertex.getGraph().getState().isGloballyTerminalState()) {
  
  			Executor executor = vertex.getGraph().getFutureExecutor();
  
  			// Only trigger if still active job
  			if (executor != null) {
  				pendingStats.add(vertex);
  
  				if (LOG.isDebugEnabled()) {
  					LOG.debug("Triggering stack trace sample for tasks: " + Arrays.toString(vertex.getTaskVertices()));
  				}
  
  				CompletableFuture<StackTraceSample> sample = coordinator.triggerStackTraceSample(
  					vertex.getTaskVertices(),
  					numSamples,
  					delayBetweenSamples,
  					MAX_STACK_TRACE_DEPTH);
  
  				sample.handleAsync(new StackTraceSampleCompletionCallback(vertex), executor);
  
  				return true;
  			}
  		}
  
  		return false;
  	}
docs: Triggers a stack trace sample for a operator to gather the back pressure
  statistics. If there is a sample in progress for the operator, the call
  is ignored.
  
  @param vertex Operator to get the stats for.
  @return Flag indicating whether a sample with triggered.