Include AvailabilityTelemetry.Timestamp as microsoft.availability.testTimestamp attribute

[Copilot]

TrackAvailability drops the user-provided Timestamp from AvailabilityTelemetry. The log record is emitted with the current wall-clock time, breaking end-to-end transaction timelines where the availability test start time precedes dependent HTTP calls.

Changes

• TelemetryClient.cs — Emit microsoft.availability.testTimestamp (ISO 8601 UTC) in the properties dictionary when Timestamp != default, using Timestamp.UtcDateTime to normalize to UTC. Consistent with the existing microsoft.availability.* attribute contract. Downstream exporters can use this to set the correct event time.
• TelemetryClientTest.cs — Added timestamp assertion to two existing tests; added TrackAvailabilityPreservesUserSpecifiedTimestamp and TrackAvailabilityOmitsTimestampWhenDefault.
• CHANGELOG.md — Added entry under Unreleased.

// Before: timestamp silently ignored
var avail = new AvailabilityTelemetry("Test", startTime, duration, "Sweden Central", true);
client.TrackAvailability(avail); // log record gets DateTimeOffset.UtcNow

// After: timestamp propagated as UTC attribute
// properties["microsoft.availability.testTimestamp"] = "2025-04-19T12:10:59.9930000Z"

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[rajkumar-rangaraj]

@copilot Add changelog update

[Copilot]

@copilot Add changelog update

Added in 95d2745.

[Copilot]

Pull request overview

This PR ensures TrackAvailability preserves a user-provided AvailabilityTelemetry.Timestamp by emitting it as a microsoft.availability.testTimestamp log attribute (ISO 8601), enabling downstream exporters to reconstruct the correct event time for availability tests.

Changes:

• Add microsoft.availability.testTimestamp attribute when AvailabilityTelemetry.Timestamp != default.
• Extend existing availability tests to assert timestamp propagation.
• Add new tests to validate timestamp presence/absence for explicit vs default timestamps.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
BASE/src/Microsoft.ApplicationInsights/TelemetryClient.cs	Adds emission of microsoft.availability.testTimestamp from AvailabilityTelemetry.Timestamp when set.
BASE/Test/Microsoft.ApplicationInsights.Test/Microsoft.ApplicationInsights.Tests/TelemetryClientTest.cs	Adds/extends tests to verify timestamp is included when provided and omitted when default.

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[Copilot]

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