Create a new set of metrics

[David Lambauer]

For now, we're having a bunch of different metrics but still have more that we want to provide.

To enhance the current functionality of the module, we'd like to add the following metrics:

• magento_shipments_count_total
• magento_catalog_category_count_total
• magento_website_count_total
• magento_store_count_total

The following labels shall always be included in each metric:

• store_code

We need to prioritize performance in our data collectors as this module is used by Magento Stores that are quite large.

Make sure to test and document the changes.