Add StmtType to error count

## Enhancement
for error rate alerts, we are currently using the query
```
sum(rate(tidb_server_query_total{tidb_cluster="xxx", result="Error"}[1m])) * 100 / sum(rate(tidb_server_query_total{tidb_cluster="xxx"}[1m])) > #{threshold}
```

however, this is a total error rate for all types of queries. We want to breakdown error rate by query types eg. (Select, Delete, Update, Insert … )
would be great to add query types into the metric `tidb_server_query_total`