code: public int availablePermits()
      {
          stateCheck(State.CREATED, State.RUNNING, State.HEALTH_CHECKING);
          return permits.availablePermits();
      }
docs: Returns the number of permits currently available.
  The number of available permits indicates how many additional
  connections can be made without blocking.
  
  @return the number of available permits.
  @see Builder#withMaxConnections(int)