kv: entireSpanExcludedFromBackupRLocked returning false

[dt]

We've observed some of our clusters failing to backup citing a gc threshold in the leases table of a tenant. This is interesting since this table has a config that says it is excluded from backup, so typically a gc error to a range for it would be decorated with a bool telling the backup processor that it can ignore the error since it didn't need data from that range anyway. However we're seeing this error bubble up which means it does not have this boolean on it. This seems like it could be because some tables, such as the MR system.leases table, have many span configs for the same table that are applying within the same range, but the entireSpanExcludedFromBackupRLocked check will report false right now if the config is not the same for the whole span (even if every config has the bit). We do see a stream of the warnings logged in this case in the logs.

More context in https://cockroachlabs.slack.com/archives/C0KB9Q03D/p1728074952547289.

Jira issue: CRDB-42779

[blathers-crl]

Hi @dt, please add branch-* labels to identify which branch(es) this C-bug affects.

_{🦉 Hoot! I am a Blathers, a bot for CockroachDB. My owner is dev-inf.}