Use constants to decide distro version in the exporter

[rads-1996]

Description

Populate the different distro names based on the naming convention.

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

Updates the exporter’s internal SDK version string generation so it can report different “distribution” identifiers based on standardized environment variable names, and adds unit tests covering the new formatting/priority rules.

Changes:

• Add SDK-version-related environment variable constants and use them in _utils._get_sdk_version().
• Extend _get_sdk_version() formatting to emit mot (Microsoft OpenTelemetry) or dst (Azure Monitor distro) when corresponding env vars are present, with mot taking priority.
• Add unit tests to validate default, distro, Microsoft OTel, and priority behaviors.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
sdk/monitor/azure-monitor-opentelemetry-exporter/tests/test_utils.py	Adds coverage for _get_sdk_version() across default/distro/Microsoft OTel scenarios and priority behavior.
sdk/monitor/azure-monitor-opentelemetry-exporter/azure/monitor/opentelemetry/exporter/_utils.py	Implements env-var-driven SDK version string variants and prioritization logic.
sdk/monitor/azure-monitor-opentelemetry-exporter/azure/monitor/opentelemetry/exporter/_constants.py	Introduces constants for SDK-version environment variable names.