pkg/server: move DataDistribution to systemAdminServer

The DataDistribution endpoint reports replica counts by
database and table. When it was built, it operated off
the assumption that a range would only ever contain a
single table's data within.

Now that we have coalesced ranges, a single range can
span multiple tables. Unfortunately, the DataDistribution
endpoint does not take this fact into account, meaning
it reports garbled and inaccurate data, unless the
`spanconfig.storage_coalesce_adjacent.enabled` setting
is set to false (see #98820).

For secondary tenants, ranges are *always* coalesced,
so this endpoint in its current state could never report
meaningful data for a tenant.

Given all of this, we have decided to make this endpoint
only available for the system tenant. This patch
accomplishes this by moving the endpoint away from the
adminServer and into the systemAdminServer, making
it effectively unimplemented for secondary tenants.

Release note: none

pkg/server/admin.go

@@ -2973,7 +2973,14 @@ func (s *systemAdminServer) Decommission(
 }
 
 // DataDistribution returns a count of replicas on each node for each table.
-func (s *adminServer) DataDistribution(
+//
+// TODO(kv): Now that we have coalesced ranges, this endpoint no longer reports
+// accurate replica counts. Furthermore, since it doesn't take coalesced ranges
+// into account, this endpoint doesn't work for secondary tenants whose ranges are
+// *always* coalesced. Update this endpoint to handle coalesced ranges and
+// implement tenant filtering, after which it can be moved back into the
+// adminServer instead of the systemAdminServer.
+func (s *systemAdminServer) DataDistribution(
 	ctx context.Context, req *serverpb.DataDistributionRequest,
 ) (_ *serverpb.DataDistributionResponse, retErr error) {
 	if err := s.requireViewClusterMetadataPermission(ctx); err != nil {