@Nullable
    private static EnumConverter<?> enumConverter(Class<?> elementType) {
        if (!elementType.isEnum()) {
            return null;
        }
        return enumConverters.get(elementType);
    }
Returns an attribute resolver which retrieves a value specified by {@code attrKeys}
from the {@link RequestContext}.
core/src/main/java/com/linecorp/armeria/internal/server/annotation/AnnotatedValueResolver.java