test(consumer): wait for transport metrics before assertions

[sbalabanov]

Summary

Intent:

• Remove a scheduling race that let consumer metrics tests snapshot before transport completion metrics were recorded.

Changes:

• Stop the consumer before taking metric snapshots so worker draining synchronizes Ack, Nack, and Reject completion.
• Preserve the existing metric assertions without sleeps, retries, or weaker expectations.

---

_{Generated by the 🪄 pr-create skill in devexp-agent-marketplace}

Test Plan

Issues

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}