Clarify and stabilize `db.response.returned_rows` and `db.client.response.returned_rows`

[lmolkova]

Extracted from #1984 (comment)

DB spans cover the time it takes to get the first (batch) of rows (and not the time it takes to iterate over it) .
So it's not usually possible to know how many rows are returned without iterating over the result.

Discussed this in DB SIG call on 3/28:

• Keep experimental for now
  • Can stabilize as needed for specific databases (but not for general databases)
    • Those databases need to define exactly what the attribute and metric mean to them
    • Can generalize it later
  • Open questions about stabilization
    • Terminology “rows” vs “records”
    • Metric: do we need a potential name for counter as well
    • What does returned rows mean for batch operations?