Fix GA4 indexes on option select search facets

[andysellick]

⚠️ This repo is Continuously Deployed: make sure you follow the guidance ⚠️

What

Fix GA4 indexes on search facets when expanded or collapsed on pages like: https://www.gov.uk/search/guidance-and-regulation

• the indexes were coming out wrong for the option select facets, increased by 1, but okay for any preceding or following facets
• looks to be a problem where the values for index_section and index_section_count are cached inside the facet and are therefore sometimes wrong
• need to pass them to the cache_key to let it invalidate the cache if the numbers change

Thanks to @sihugh for invaluable cache advice 👍

Why

The indexes were being generated as e.g.

1. (topic facet) index_section: 1, index_section_count: 4
2. (option select facet) index_section: 3, index_section_count: 5 (wrong)
3. (option select facet) index_section: 4, index_section_count: 5 (wrong)
4. (date facet) index_section: 4, index_section_count: 4

Visual changes

None.

Trello card: https://trello.com/c/l5YsyfsN/794-fix-incorrect-index-values-on-guidance-and-regulation-finder-filters

[sihugh]

Looks good to me 👍