Fix checkpoint_sync_time value type

Error:
sql: Scan error on column index 3, name \"checkpoint_sync_time\": converting driver.Value type float64 (\"1.876469e+06\") to a int: invalid syntax

See also:
#633
#666

Signed-off-by: Nicolas Rodriguez <nico@nicoladmin.fr>

collector/pg_stat_bgwriter.go

@@ -127,7 +127,7 @@ func (PGStatBGWriterCollector) Update(ctx context.Context, server *server, ch ch
 	var cpt int
 	var cpr int
 	var cpwt float64
-	var cpst int
+	var cpst float64
 	var bcp int
 	var bc int
 	var mwc int