code: void retain(Object owner, Field field) {
          retain();
          Map<String, Integer> fields = owners.get(owner);
          if (fields == null) {
              fields = new HashMap<>();
              owners.put(owner, fields);
          }
  
          Integer count = fields.get(field.toGenericString());
          if (count == null) {
              fields.put(field.toGenericString(), 1);
          } else {
              count++;
              fields.put(field.toGenericString(), count);
          }
      }
docs: Retain an instance injected as a field of an object
  @param owner The owner of the field
  @param field The field