pg_catalog.pg_relation_size()

[Karitham]

Is your feature request related to a problem? Please describe.
Not having pg_catalog.pg_relation_size() breaks usql's introspection queries.

Describe the solution you'd like
Adding pg_relation_size

Describe alternatives you've considered
Trying to replace usql's usage of pg_relation_size with a cockroach specific implementation for metadata and instrumentation.

Additional context
https://github.com/xo/usql/blob/master/drivers/metadata/postgres/metadata.go The ideal output would be supporting the queries built here, to support all these features.

Jira issue: CRDB-18527

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[rafiss]

Thanks for the issue! #20712 is a similar issue where we've been tracking this work.

[knz]

After #103128 is completed, there will be a new built-in function able to compute fine grained MVCC stats per KV span. We will be able to combine this with the index_span() builtin to implement pg_relation_size and the other size built-in functions.