[Enhancement] SQL: Chats, Average number of messages in a chat

**MODULE**: Analytics
**SCOPE**: Chats
**SQL**: DSL/Resql/analytics/POST/chat-avg-num-of-messages.sql

### Acceptance Criteria

- [ ] The chat needs to have an ENDED status.
- [ ] Only average number is used (as-is).
- [ ] All Test chat messages are excluded (as-is).
