Fix model field defined filter labels fetching for schema

[meteozond]

• 1. There is automatic filter label fetching from model fields based on Filter.model attribute.
• 2. Filter.model attribute is set by FilterSet.__init__() so it requires instantiation.
• 3. RestFrameworkFilterBackend.get_schema_operation_parameters uses FilterSet class instead of instance.

To fix RestFrameworkFilterBackend.get_schema_operation_parameters we need FilterSet instance inspection instead of class.

[carltongibson]

Can you add a test case for this? Thanks.

[meteozond]

I'm not sure that it deserves separate test. Is modified existing GetSchemaOperationParametersTests test enough?

[carltongibson]

As per #1432 the in-built schema support is going to be deprecated in the next version.

Rather, I recommend you migrate to use drf-spectacular, which is much more fully featured, and under active development.

https://drf-spectacular.readthedocs.io/en/latest/readme.html

If for some reason you're not able to use drf-spectacular you can copy the django_filters.rest_framework.backends.DjangoFilterBackend into your own project to customise as needed. (But use drf-spectacular.)

Thanks.