fix for exporter issue 633

fix for exporter issue 633: #633

"Scan error on column index 2, name \"checkpoint_write_time\": converting driver.Value type float64 (\"6.594096e+06\") to a int: invalid syntax #633"

Signed-off-by: bravosierrasierra <bravosierrasierra@users.noreply.github.com>

Author: bravosierrasierra

collector/pg_stat_bgwriter.go

@@ -122,7 +122,7 @@ func (PGStatBGWriterCollector) Update(ctx context.Context, db *sql.DB, ch chan<-
 
 	var cpt int
 	var cpr int
-	var cpwt int
+	var cpwt float64
 	var cpst int
 	var bcp int
 	var bc int