Describe the problem

Currently SHOW RANGES WITH DETAILS only includes range sizes, not object sizes.

When a range contains multiple table/indexes, the range size cannot be used to estimate the table/index size any more.

So we need a replacement for where people would previously use SUM on the range size.

This can use the new span stats computation that Zach implemented in #96223 .

Jira issue: CRDB-24925
Epic: CRDB-24928