Add statistics on the total number of topics, the maximum number of subscriptions, and the maximum number of unacknowledged subscriptions

[HarryFQG]

(If this PR fixes a github issue, please add Fixes #<xyz>.)

Fixes:

Add statistics on the total number of topics, the maximum number of subscriptions, and the maximum number of unacknowledged subscriptions.

Motivation

Explain here the context, and why you're making that change. What is the problem you're trying to solve.

Modifications

The main modification is in the front-end part, which aggregates the number of topic entries, the maximum number of backlogs in all subscriptions, and the number of unacknowledged items.

Verifying this change

1. Send a message to check whether the number of'All entries' is correct;
2. Increase subscriptions without consumption, and continue to produce messages to check the backlog backlog;
3. Do not ack when consuming, check the quantity of'Unack msg'.

• [ ] Make sure that the change passes the ./gradlew build checks.