[D1] Test an index should call-out the rows_read meta property

[KianNH]

Existing documentation URL(s)

• https://developers.cloudflare.com/d1/build-with-d1/use-indexes/#test-an-index

What changes are you suggesting?

Users are directed to use indexes for cost optimization, it'd be helpful if users who land on the indexes page directly are told how to test how effective their indexes are for their workload.

A good example is under the "How can I estimate my (eventual) bill?" heading on https://developers.cloudflare.com/d1/platform/pricing/#frequently-asked-questions

Additional information

No response

[KimJ15]

This issue is pretty stale. Closing as won't do for now.

[Oxyjun]

Retrospectively actioning this to close internal ticket. Linked relevant FAQ section.

#18169