Need some monitors for Multi-Statement

[jackysp]

Enhancement

The multi-statement feature allows the clients to send SQLs in batch, which sometimes leads the users confused, such as, they will see the command per second is low and the QPS is high, and also they will see some SQLs will have a bad long tail latency, but they could not find the slow query in tidb-slow-query files. It is caused by the whole latency calculated as the last SQL one which is in a multi-statement.

It's better to add a metrcis or something else for multi-statemnt.