sql/stats: generate statistics forecasts in the stats cache

As of this commit, we now try to generate statistics forecasts for every
column of every table. This happens whenever statistics are loaded into
or refreshed in the stats cache. We use only the forecasts that fit the
historical collected statistics very well, meaning we have high
confidence in their accuracy.

Fixes: #79872

Release note (performance improvement): Enable table statistics
forecasts, which predict future statistics based on historical collected
statistics. Forecasts help the optimizer produce better plans for
queries that read data modified after the latest statistics collection.
We use only the forecasts that fit the historical collected statistics
very well, meaning we have high confidence in their accuracy. Forecasts
can be viewed using `SHOW STATISTICS FOR TABLE ... WITH FORECAST`.

pkg/sql/stats/automatic_stats_test.go

@@ -773,8 +773,14 @@ func checkStatsCount(
 		if err != nil {
 			return err
 		}
-		if len(stats) != expected {
-			return fmt.Errorf("expected %d stat(s) but found %d", expected, len(stats))
+		var count int
+		for i := range stats {
+			if stats[i].Name != jobspb.ForecastStatsName {
+				count++
+			}
+		}
+		if count != expected {
+			return fmt.Errorf("expected %d stat(s) but found %d", expected, count)
 		}
 		return nil
 	})

pkg/sql/stats/stats_cache.go

@@ -720,5 +720,8 @@ ORDER BY "createdAt" DESC, "columnIDs" DESC, "statisticID" DESC
 		return nil, err
 	}
 
+	forecasts := ForecastTableStatistics(ctx, statsList)
+	statsList = append(forecasts, statsList...)
+
 	return statsList, nil
 }