Use constants to decide distro version

[rads-1996]

Description

Pass the azure monitor distro version the exporter to set the telemetry sdk version, which is then passed on application insights.

All SDK Contribution checklist:

• The pull request does not introduce [breaking changes]
• CHANGELOG is updated for new features, bug fixes or other significant changes.
• I have read the contribution guidelines.

General Guidelines and Best Practices

• Title of the pull request is clear and informative.
• There are a small number of commits, each of which have an informative message. This means that previously merged commits do not appear in the history of the PR. For more information on cleaning up the commits in your PR, see this page.

Testing Guidelines

• Pull request includes test coverage for the included changes.

[Copilot]

Pull request overview

This PR aims to propagate the Azure Monitor distro version via configuration so downstream components (notably the exporter / telemetry pipeline) can identify the distro version consistently.

Changes:

• Adds a distro-version env var write during _get_configurations().
• Re-exports a new constant from the exporter constants module to avoid duplicating the env var name.
• Adds a unit test asserting the env var is set during configuration.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 4 comments.

File	Description
sdk/monitor/azure-monitor-opentelemetry/azure/monitor/opentelemetry/_constants.py	Imports/re-exports exporter constants intended to standardize distro version keys.
sdk/monitor/azure-monitor-opentelemetry/azure/monitor/opentelemetry/_utils/configurations.py	Sets the distro version env var when building configurations.
sdk/monitor/azure-monitor-opentelemetry/tests/utils/test_configurations.py	Adds coverage to validate the env var gets set.

[Copilot]

Setting the distro version into os.environ here overwrites any existing value each time _get_configurations() is called. If the intent is just to provide a default for downstream components, consider using environ.setdefault(...) (similar to _default_resource) so user/host-provided values aren’t clobbered.

Suggested change

	environ[_AZURE_MONITOR_DISTRO_VERSION] = VERSION
	environ.setdefault(_AZURE_MONITOR_DISTRO_VERSION, VERSION)