protected <E extends Annotation> void registerParameterAnnotation(
      Class<E> annotation, DeclarativeContract.ParameterAnnotationProcessor<E> processor) {
    this.parameterAnnotationProcessors.put(
        (Class) annotation, (DeclarativeContract.ParameterAnnotationProcessor) processor);
  }
Called while method parameter annotations are being processed

@param annotation to be processed
@param processor function that defines the annotations modifies {@link MethodMetadata}
core/src/main/java/feign/DeclarativeContract.java