If the current active session exceeds max_active_session, consider allowing the administrator account to connect to the query

[TCeason]

Example:

./target/debug/databend-query --meta-endpoints=127.0.0.1:9191 --mysql-handler-port=3307 --clickhouse-http-handler-port=8124 --http-handler-port=8000 --metric-api-address=127.0.0.1:7071 --admin-api-address=127.0.0.1:8081 --flight-api-address=127.0.0.1:9091
 --max-active-sessions=1

If we start a query with max-active-sessions = n, and then the n session will execute a long query or a big query.

So now we can't manage the query.

Should we support some extra_conn and extra_port like this: https://mariadb.com/docs/skysql/ref/mdb/system-variables/extra_max_connections/