Allow environment variables for exportarr

Description

Allow for PROWLARR__BACKFILL - and then by default enable any exportarr environment variable for config.

Also, disable some metrics by default which shouldn't have been enabled by default!

[x] Bug fix (non-breaking change which fixes an issue)
[x] Non-breaking change which adds functionality
[ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
[ ] Documentation update

[x] I have updated the chart version in Chart.yaml according to semantic versioning.
[x] I have included any new or changed values in the values.yaml file and documented them in the README if applicable.
[x] My changes are tested and proven to work.
[x] I have performed a self-review of my own code.
[x] I have commented my code, particularly in hard-to-understand areas.
[x] I have made corresponding changes to the documentation.
[x] My changes generate no new warnings.