Provide recipe index health diagnostics

[jayaddison]

Is your feature request related to a problem? Please describe.
We typically re-crawl and re-index recipe content after changes to our product data and upgrades to our crawling pipeline.

It can be difficult to determine how 'fresh' our recipe index is currently; the indexed_at column in the database is probably the best way to assess this.

Describe the solution you'd like
It'd be nice to have a chart in the diagnostics dashboard that displays a histogram of indexed recipes by day or month.

Describe alternatives you've considered
This could be performed as a query on-demand when needed.

Additional context
The OpenSearch query for this looks something like:

{
    "aggs": {
        "documents_by_date": {
            "date_histogram": {"field": "indexed_at", "calendar_interval": "day"}
        }
    }
}