fix(network): prevent metric leak in outgoing message queue on session teardown

[prestoalvarez]

Summary

Fixes a metric leak in QueuedOutgoingMessages where the Gauge counter could become permanently inflated when sessions terminate with messages still queued.

Problem

The Gauge metric was only decremented on explicit pop_front() calls, but never when the queue was destroyed. If a session terminated with pending messages in the queue, the metric would remain artificially high indefinitely, causing:

• Inaccurate monitoring data
• Potential false alerts
• Difficulty in diagnosing queue health

Solution

Added a Drop implementation for QueuedOutgoingMessages that decrements the gauge by the number of remaining messages when the struct is destroyed.