feat: show flow's mem usage in INFORMATION_SCHEMA.FLOWS (#4890)

* feat: add flow mem size to sys table

* chore: rm dup def

* chore: remove unused variant

* chore: minor refactor

* refactor: per review

tests/cases/standalone/common/flow/flow_basic.sql

@@ -569,6 +569,12 @@ SELECT
 FROM
     requests_without_ip;
 
+-- Test if FLOWS table works, but don't care about the result since it vary from runs
+SELECT
+    count(CASE WHEN state_size > 0 THEN 1 ELSE 0 END) as active_flows,
+FROM
+    INFORMATION_SCHEMA.FLOWS;
+
 DROP FLOW requests_long_term;
 
 DROP TABLE requests_without_ip;