Fix queue diff metrics for disabled clusters

[yycptt]

What changed?

• Ignore disabled cluster when calculating queue diff metrics

Why?

• Disabled clusters' ack level won't advance and they also won't prevent overall task queue ack level from advancing. If they are included in the calculation, the queue diff metrics will only keeping increasing, while there's actually no lag at all.

How did you test it?
Existing tests

Potential risks
N/A, just changing metrics

Is hotfix candidate?
No.