Computed field properties do not appear in JSON:API responses unless their definition explicitly calls setInternal(FALSE)

Problem/Motivation

I am using the Serial module (https://drupal.org/project/serial) which provides an auto-increment field type. When making a jsonapi request for bundles with serial fields, the field is present in the response, but the value is missing. An issue was filed on that project, but is stuck. See https://www.drupal.org/project/serial/issues/2921349.

The heart of the issue is how modules that provide FieldType plugins should interact with jsonapi. Currently, the advice has been to:

1) decorate a normalizer service
2) add a @DataType normalizer
3) write an enhancer using jsonapi_extras

I'm finding very little in the way of useful documentation or examples for how to do these things, and I haven't come up with any solution other than to hack the jsonapi module itself.

There are a lot of modules that provide @FieldType plugins, and not @DataType plugins. In the case of the Serial module, the field type is a computed field. In \Drupal\jsonapi\Normalizer\FieldItemNormalizer::normalize(), this line

$field_properties = TypedDataInternalPropertiesHelper::getNonInternalProperties($field_item);

does not return an array of properties for the Serial field because it's a computed field. I feel like there may be a number of other modules that won't work with jsonapi for similar reasons.

Proposed resolution

My hack checks specifically for SerialItem fields to get the value:

diff --git a/src/Normalizer/FieldItemNormalizer.php b/src/Normalizer/FieldItemNormalizer.php
index 618d97e..fd7f50a 100644
--- a/src/Normalizer/FieldItemNormalizer.php
+++ b/src/Normalizer/FieldItemNormalizer.php
@@ -66,6 +66,9 @@ class FieldItemNormalizer extends NormalizerBase implements DenormalizerInterfac
     if (!empty($field_item->getProperties(TRUE))) {
       // We normalize each individual value, so each can do their own casting,
       // if needed.
+      if ($field_item instanceof \Drupal\serial\Plugin\Field\FieldType\SerialItem) {
+        $values = $field_item->get('value')->getValue();
+      }
       $field_properties = TypedDataInternalPropertiesHelper::getNonInternalProperties($field_item);
       foreach ($field_properties as $property_name => $property) {
         $values[$property_name] = $this->serializer->normalize($property, $format, $context);

This patch shows what I'm trying to accomplish, and what I need is a way to do this without hacking up the module. I need some clear guidance regarding how to solve this problem - something that can be documented for other module maintainers who encounter similar issues.