private static Object convert(String value, Class<?> elementType,
                                  @Nullable EnumConverter<?> enumConverter) {
        return enumConverter != null ? enumConverter.toEnum(value)
                                     : stringToType(value, 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