As a plugin developer I need configurable MIDDLEWARE list setting

Author: osapryki (osapryki)

Redmine Issue: 6467, https://pulp.plan.io/issues/6467

In actual pulp implementation it is only possible to append values to MIDDLEWARE django setting or completely overriding it.

In some cases inserting value at the beginning of the MIDDLEWARE list may be required. An example of such case is using django_prometheus for adding prometheus metrics endpoint. This plugins requires inserting one middleware class to the beginning of MIDDLEWARE list and second one to the end.

Unfortunately this is not possible without completely overriding MIDDLEWARE list with custom list that has to include all middleware classes copied from pulpcore settings. It may lead to issues in case when the default list in pulpcore repository is updated and has to be synchronized with all plugins that override it in such way.

Propositions:

1. Move contents of MIDDLEWARE list to DEFAULT_MIDDLEWARE and move it to module default_settings.py, and make it importable and therefore re-usable by plugin developers.

2. Provide some sort of hooks to inject values to MIDDLEWARE list, for example but not necessary based on plugin entry_points.

MIDDLEWARE may be not the only parameter that requires such customization in future, generic approach to this problem may be useful for long term.

Start by reviewing the MIDDLEWARE setting and the proposed default_settings.py location. Compare the two suggested approaches—an importable DEFAULT_MIDDLEWARE or injection hooks such as entry_points—and determine which provides safe plugin customization without copying pulpcore defaults. Done should include an agreed generic approach and coverage for middleware configuration.