docs: public Map<TopologyAPI.StreamId, TopologyAPI.Grouping> getSources(String componentId) {
      if (inputs.containsKey(componentId)) {
        Map<TopologyAPI.StreamId, TopologyAPI.Grouping> retVal =
            new HashMap<>();
        for (TopologyAPI.InputStream istream : inputs.get(componentId)) {
          retVal.put(istream.getStream(), istream.getGtype());
        }
        return retVal;
      } else {
        return null;
      }
    }