Fix: Cloud RoleName/Instance update applies to all telemetry

[rajkumar-rangaraj]

Issue

Setting TelemetryClient.Context.Cloud.RoleName after construction has no effect.

In 2.x, customers could set the cloud role name at any time:

var client = new TelemetryClient(config);
client.Context.Cloud.RoleName = "MyWebApp"; // worked in 2.x, silently ignored in 3.x

In 3.x, Context.Cloud.RoleName is read during the constructor (ApplyCloudContextToResource()), but at that point the value is still null. The OTel Resource is then built and becomes immutable — any value set afterward is ignored.

Fix

CloudContext.RoleName and CloudContext.RoleInstance setters now write to well-known environment variables:

• APPLICATIONINSIGHTS_CLOUD_ROLE_NAME
• APPLICATIONINSIGHTS_CLOUD_ROLE_INSTANCE

The Azure Monitor Exporter reads these in SetResourceSdkVersionAndIkey() (the single method where cloud role tags are stamped on every telemetry envelope) and overrides the immutable Resource value. The env var is read once and cached — zero ongoing cost for customers who don't use this feature.

Requires companion PR in azure-sdk-for-net for the exporter-side change.

Follow up

• Set CloudContext to public.

Checklist

• I ran Unit Tests locally.
• CHANGELOG.md updated with one line description of the fix, and a link to the original issue if available.

For significant contributions please make sure you have completed the following items:

• Design discussion issue #
• Changes in public surface reviewed

The PR will trigger build, unit tests, and functional tests automatically. Please follow these instructions to build and test locally.

Notes for authors:

• FxCop and other analyzers will fail the build. To see these errors yourself, compile localy using the Release configuration.

[Copilot]

Pull request overview

This PR attempts to fix a regression in 3.x where setting TelemetryClient.Context.Cloud.RoleName or RoleInstance after TelemetryClient construction has no effect. The solution writes these values to environment variables that the Azure Monitor Exporter can read at export time, working around the immutable OpenTelemetry Resource. This requires a companion PR in azure-sdk-for-net.

Changes:

• Modified CloudContext setters to write to environment variables when RoleName/RoleInstance are set
• Added a test verifying environment variables are set correctly
• Updated CHANGELOG to document the fix

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 5 comments.

File	Description
CHANGELOG.md	Documents the fix with an incomplete PR link
CloudContext.cs	Modified RoleName and RoleInstance setters to call Environment.SetEnvironmentVariable
TelemetryClientTest.cs	Added test verifying environment variables are set when CloudContext properties are assigned

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