reports: generate replication reports for secondary tenants

[irfansharif]

#66348 outlined a scheme to support zone configs for secondary tenants; we've implemented an MVP as part of #67679. One thing we skipped was replication reports and generating them for secondary tenants. The code generating these reports today sits under pkg/kv/kvserver/reports, and is very tightly coupled to the system config span (so it only works for the system tenant). We want to generate these same reports for secondary tenants. Probably we can hoist all that code out of pkg/kv, strip out the dependency on config.SystemSystem, generate them within the pods by power it using a view over system.{descriptor,zones}. To generate these reports, we need access to store descriptors and node liveness; we could expose RPCs through the Connector interface to retrieve these. (We should probably only export the portions relevant to these reports.)

Jira issue: CRDB-10117

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[irfansharif]

This issue covers two concerns:
a) Using the new span configs infrastructure to power replication reports for the host tenant;
b) Generating span-configs-infrastructure-powered reports for secondary tenants.

@ajwerner thought of with this cute hack to do (a), non-invasively swapping out the reports dependency on the system config span in favor of a spanconfig.Store (#70287): when translating (#71359) zone configs to span configs, we could also include the descriptor ID and partition index as part of the spanconfig proto. In our report generation code, with a conveniently populated spanconfig.Store, we could then just reverse-lookup the span config for a given descriptor ID.

cockroach/pkg/kv/kvserver/reports/replication_stats_report.go

Line 308 in d2017ce

zone, err := getZoneByID(i, v.cfg)

[irfansharif]

#89987 (comment)