How to implement readHandler on customer

Question

I am trying to implement a ReadHandler on customer. Based on the cms module page read handler I did the following.

In my module etc/di.xml

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
    <type name="Magento\Framework\EntityManager\MetadataPool">
        <arguments>
            <argument name="metadata" xsi:type="array">
                <item name="Magento\Customer\Api\Data\CustomerInterface" xsi:type="array">
                    <item name="entityTableName" xsi:type="string">customer_entity</item>
                    <item name="identifierField" xsi:type="string">entity_id</item>
                </item>
            </argument>
        </arguments>
    </type>
    <type name="Magento\Framework\EntityManager\Operation\ExtensionPool">
        <arguments>
            <argument name="extensionActions" xsi:type="array">
                <item name="Magento\Customer\Api\Data\CustomerInterface" xsi:type="array">
                    <item name="read" xsi:type="array">
                        <item name="customerReader" xsi:type="string">MyCompany\MyModuleCore\Model\ResourceModel\Customer\Relation\Privacy\ReadHandler</item>
                    </item>
                </item>
            </argument>
        </arguments>
    </type>

    <type name="Magento\Framework\EntityManager\HydratorPool">
        <arguments>
            <argument name="hydrators" xsi:type="array">
                <item name="Magento\Customer\Api\Data\CustomerInterface" xsi:type="string">Magento\Framework\EntityManager\AbstractModelHydrator</item>
            </argument>
        </arguments>
    </type>

</config>

and in

MyCompany\MyModuleCore\Model\ResourceModel\Customer\Relation\Privacy\ReadHandler.php file:

use Magento\Customer\Model\Customer;
use Magento\Framework\EntityManager\MetadataPool;
use Magento\Framework\EntityManager\Operation\ExtensionInterface;

class ReadHandler implements ExtensionInterface
{

    /**
     * @var MetadataPool
     */
    protected $metadataPool;

    /**
     * @var Customer
     */
    protected $resourceCustomer;

    public function __construct(
        MetadataPool $metadataPool,
        Customer $resourceCustomer
    )
    {
        $this->metadataPool = $metadataPool;
        $this->resourceCustomer = $resourceCustomer;
    }

    public function execute($entity, $arguments = [])
    {
        $this->metadataPool->getMetadata();
        return $entity;
    }


}

I've put breakpoints into the constructor and into the execute method but neither runs. Debugger works.

I've checked that the Model/Customer.php does not implements CustomerInterface but the Model/Data/Customer.php does. I've set breakpoint there and debugger stops there.

What do I missing?