code: public Optional<ServerPort> activePort() {
          synchronized (activePorts) {
              return Optional.ofNullable(Iterables.getFirst(activePorts.values(), null));
          }
      }
docs: Returns the primary {@link ServerPort} that this {@link Server} is listening to. This method is useful
  when a {@link Server} listens to only one {@link ServerPort}.
  
  @return {@link Optional#empty()} if this {@link Server} did not start