[Enhancement] Add metric for last delayed message timestamp

[Dream95]

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• I searched in the issues and found nothing similar.

Motivation

Without knowing when the last delayed message is scheduled to be delivered, we cannot easily:

1. Monitor how far in the future delayed messages extend (e.g. "last message is 7 days from now")
2. Alert when delayed messages are scheduled too far ahead (e.g. misconfigured producers)
3. Debug delayed delivery issues (e.g. why consumers are not receiving messages yet)
4. Plan capacity (e.g. understanding the time range of pending delayed messages)

Solution

Add a new metric: the delivery timestamp of the last delayed message .
Name: lastDelayedMessageTimestamp

Alternatives

No response

Anything else?

No response

Are you willing to submit a PR?

• I'm willing to submit a PR!