Swap out service for a decorator

service-decorator.php

<?php

/**
 * @file
 * Contains Drupal\my_module\MyModuleServiceProvider
 */

namespace Drupal\my_module;

use Drupal\Core\DependencyInjection\ContainerBuilder;
use Drupal\Core\DependencyInjection\ServiceProviderBase;

/**
 * Modifies the language manager service.
 */
class MyModuleServiceProvider extends ServiceProviderBase {

  /**
   * {@inheritdoc}
   */
  public function alter(ContainerBuilder $container) {
    // Overrides entity.manager service for our own.
    $currentService = $container->get('entity.manager');
    $container->set('entity.manager', new MyEntityManagerDecorator($currentService));
  }
}