Clarify USM environment variable detection limitations

[ddOfir]

Add documentation explaining that Universal Service Monitoring (USM) only detects environment variables set at process start time, not those set programmatically within application code.

This clarifies that while APM instrumentation libraries can read DD_SERVICE and similar variables set via System.setProperty() (Java) or Environment.SetEnvironmentVariable() (.NET), USM cannot detect these as it reads from /proc/PID/environ (Linux) or system APIs (Windows) which only contain the initial process environment.

Includes examples for properly setting environment variables in Docker, Kubernetes, and shell environments to ensure USM detection.

What does this PR do? What is the motivation?

Add documentation explaining that Universal Service Monitoring (USM) only detects environment variables set at process start time, not those set programmatically within application code

Merge instructions

Merge readiness:

• Ready for merge

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Additional notes

[github-actions]

Preview links (active after the build_preview check completes)

Modified Files

• https://docs-staging.datadoghq.com/ofir.yefet/usm-env-var-detection-clarification/universal_service_monitoring/setup

[maycmlee]

Some suggestions

[maycmlee]

Suggested change

	<strong>Important:</strong> Universal Service Monitoring detects service names from environment variables that exist when a process starts. USM reads these values from the operating system (from <code>/proc/PID/environ</code> on Linux or via system APIs on Windows).
	<strong>Important:</strong> Universal Service Monitoring detects service names from environment variables that exist when a process starts. USM reads these values from the operating system (from <code>/proc/PID/environ</code> on Linux or using system APIs on Windows).

[maycmlee]

Suggested change

	Setting environment variables programmatically **inside your application code** (such as `System.setProperty("dd.service", "my-service")` in Java or `Environment.SetEnvironmentVariable("DD_SERVICE", "my-service")` in .NET) will **not** be detected by USM, even though these values work for APM tracing instrumentation.
	Environment variables programmatically set **inside your application code** (such as `System.setProperty("dd.service", "my-service")` in Java or `Environment.SetEnvironmentVariable("DD_SERVICE", "my-service")` in .NET) are **not** detected by USM, even though these values work for APM tracing instrumentation.

[maycmlee]

Suggested change

	**Why this happens:**
	**Why USM does not detect environment variables that are set programmatically:**

[cswatt]

Resolving merge conflict (these changes were also in #32723 leading to some conflict)

[cswatt]

After resolving merge conflicts, changes from #32723 are now applied, and this branch has no differences from master. Closing this PR :)