fix(metrics): emit init duration metric on platform report as fallback

[shreyamalpani]

Overview

The init duration metric is sent on the PlatformInitReport event. In the case of this event not being received / arriving late, this PR adds a fallback to set the metric using the init duration from the PlatformReport event on cold start invocations in On Demand mode.

Motivation

Saw failures on emitting the init duration metric in E2E tests

[datadog-prod-us1-4]

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⚠️ Warnings

🚦 2 Pipeline jobs failed

DataDog/datadog-lambda-extension | e2e-test-status (amd64)     

DataDog/datadog-lambda-extension | e2e-test-status (amd64, fips)     

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[Copilot]

Pull request overview

This PR ensures the init duration enhanced metric is emitted reliably on on-demand cold starts by adding a platform.report-based fallback when platform.initReport is missing or arrives late, while preventing double emission.

Changes:

• Track whether the init duration metric has already been emitted for the current sandbox (init_duration_metric_emitted).
• Emit init duration from platform.report’s init_duration_ms as a fallback when needed, and skip subsequent emission from platform.initReport.
• Add targeted tests covering emission from init report, fallback from report, warm invocations, and double-emission prevention.