DIGIT-2819 Add Queue Time Metrics for Elastic APM

[gustoliveira]

Summary

Implements queue time tracking to measure time requests spend waiting before Rails processes them. Essential for identifying capacity bottlenecks.

Queue time = X-Request-Start (NGINX) → Rails processing start - network transfer time

Background

Initial queue time functionality was cherry-picked from a previous implementation that stopped working. The stacktrace bug was fixed in a separate PR, and this PR adds improvements on top.

Bug Fixes:

• Fixed stacktrace serialization when nil or empty

Changes

New Features:

• RequestMetrics class - Calculates queue time from request headers
• Queue Time Span in APM timeline
• Transaction labels: queue_time_ms, network_time_ms, request_id

DIGIT-2819 TICKET

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