code: public void trackRequest(String requestDescriptor, long requestId) {
          Preconditions.checkNotNull(requestDescriptor, "Attempting to track a null request descriptor.");
          if (!tracingEnabled) {
              return;
          }
  
          synchronized (lock) {
              List<Long> requestIds = ongoingRequests.getIfPresent(requestDescriptor);
              if (requestIds == null) {
                  requestIds = Collections.synchronizedList(new ArrayList<>());
              }
  
              requestIds.add(requestId);
              ongoingRequests.put(requestDescriptor, requestIds);
          }
  
          log.debug("Tracking request {} with id {}.", requestDescriptor, requestId);
      }
docs: Adds a request descriptor and request id pair in the cache if tracing is enabled. In the case of tracking a
  request with an existing descriptor, this method adds the request id to the list associated to the descriptor.
  
  @param requestDescriptor Request descriptor as a single string.
  @param requestId Request id associated to the given descriptor.