server: use system stats view for stats overview queries

This commit changes the table used for the combined
stats endpoint from the view combining persisted and
in-memory stats to the view that is just a wrapper
around the system table. Thus, we are now reading
only persisted stats from the system table for the
combined stats endpoint.

Epic: None
Release note: None

pkg/server/combined_statement_stats.go

@@ -226,7 +226,7 @@ SELECT
     max(aggregated_ts) as aggregated_ts,
     metadata,
     crdb_internal.merge_statement_stats(array_agg(statistics)) AS statistics
-FROM crdb_internal.statement_statistics %s
+FROM crdb_internal.statement_statistics_persisted %s
 GROUP BY
     fingerprint_id,
     transaction_fingerprint_id,
@@ -329,7 +329,7 @@ SELECT
     fingerprint_id,
     metadata,
     crdb_internal.merge_transaction_stats(array_agg(statistics)) AS statistics
-FROM crdb_internal.transaction_statistics %s
+FROM crdb_internal.transaction_statistics_persisted %s
 GROUP BY
     app_name,
     fingerprint_id,