how to extend existing drush command class

Can anyone please let me know how to extend existing drush command class.

I am writing a custom module which provides custom content entity and created a devel generate plugin similar to ContentDevelGenerate.

It is working as expected to generate dummy content from UI . I would like to create a drush command as well for the devel generator plugin. Similar to DevelGenerateCommands content method I would like to add additional method to generate my custom content entity.

My drush command class as follows :

<?php

namespace Drupal\omdb_api\Commands;

use Drupal\devel_generate\Commands\DevelGenerateCommands;

/**
 * Class to create omdb api entity drush commands.
 */
class OmdbApiEntityDevelGenerateCommands extends DevelGenerateCommands {

  /**
   * Create omdb api entity items by drush command.
   *
   * @command devel-generate:omdb-api
   * @aliases dgen:omdb-api, devel-generate-omdb-api
   * @pluginId omdb_api_entity_devel_generate
   * @validate-module-enabled omdb_api
   *
   * @param int $num
   *   Number of omdb api entity items to generate.
   * @param array $options
   *   Array of options as described below.
   *
   * @option kill Delete all omdb api entity items before generating new omdb api entity.
   * @option feedback An integer representing interval for insertion rate logging.
   * @option skip-fields A comma delimited list of fields to omit when generating random values.
   * @option languages A comma-separated list of language codes
   */
  public function omdbApiEntities($num = 50, array $options = ['kill' => FALSE, 'feedback' => 1000]) {
    $this->generate();
  }

  /**
   * Wrapper for calling the plugin instance generate function.
   */
  public function generate() {
    $instance = $this->getPluginInstance();
    $instance->generate($this->getParameters());
  }

}

OmdbApiEntityDevelGenerateCommands If i paste this method directly in DevelGenerateCommands class, command is working as expected. So it seems I am missing something to extend the DevelGenerateCommands class.

# drush.services.yml
services:
  omdb_api.devel_generate_commands:
    class: \Drupal\omdb_api\Commands\OmdbApiEntityDevelGenerateCommands
    tags:
      - { name: develgenerate.command }