code: public ServiceInstanceQuery getEqualQueryCriterion(String key, String value){
          QueryCriterion c = new EqualQueryCriterion(key, value);
          addQueryCriterion(c);
          return this;
      }
docs: Get a metadata value equal QueryCriterion.
  
  Add a QueryCriterion to check ServiceInstance has metadata value
  equal to target.
  
  @param key
  the metadata key.
  @param value
  the metadata value should equal to.
  @return
  the ServiceInstanceQuery.