code: public RequestTag getRequestTagFor(String requestDescriptor) {
          Preconditions.checkNotNull(requestDescriptor, "Attempting to get a null request descriptor.");
          if (!tracingEnabled) {
              return new RequestTag(requestDescriptor, RequestTag.NON_EXISTENT_ID);
          }
  
          long requestId;
          List<Long> descriptorIds;
          synchronized (lock) {
              descriptorIds = ongoingRequests.getIfPresent(requestDescriptor);
              requestId = (descriptorIds == null || descriptorIds.size() == 0) ? RequestTag.NON_EXISTENT_ID : descriptorIds.get(0);
              if (descriptorIds == null) {
                  log.debug("Attempting to get a non-existing tag: {}.", requestDescriptor);
              } else if (descriptorIds.size() > 1) {
                  log.debug("{} request ids associated with same descriptor: {}. Propagating only first one: {}.",
                          descriptorIds, requestDescriptor, requestId);
              }
          }
  
          return new RequestTag(requestDescriptor, requestId);
      }
docs: Retrieves a {@link RequestTag} object formed by a request descriptor and request id. If the request descriptor
  does not exist or tracing is disabled, a new {@link RequestTag} object with a default request id is returned. In
  the case of concurrent requests with the same descriptor, multiple request ids will be associated to that request
  descriptor. The policy adopted is to retrieve the first one that was stored in the cache. Given that tracing is
  applied to idempotent operations, this allows us to consistently trace the operation that actually changes the
  state of the system. The rest of concurrent operations will be rejected and their response will be logged with a
  different requestId, as an indicator that another client request was ongoing. For more information, we refer to
  this PDP: https://github.com/pravega/pravega/wiki/PDP-31:-End-to-end-Request-Tags
  
  @param requestDescriptor Request descriptor as a single string.
  @return Request descriptor and request id pair embedded in a {@link RequestTag} object.