Fix the format of the fixed percentage sampler constant and ensure backward compatability

[rads-1996]

Description

Fix the formatting for the fixed percentage sampler to be consistent with other languages.

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.

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• 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.

[Copilot]

Pull request overview

This PR updates the format of the fixed percentage sampler constant from microsoft.fixed.percentage (with dots) to microsoft.fixed_percentage (with underscores) to be consistent with other languages like the microsoft.rate_limited sampler format. The change maintains backward compatibility by accepting both formats.

Changes:

• Updated the constant value from microsoft.fixed.percentage to microsoft.fixed_percentage
• Added backward compatibility support to accept both formats
• Updated documentation and samples to reflect the new format
• Added a test case to verify backward compatibility works correctly

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
sdk/monitor/azure-monitor-opentelemetry/azure/monitor/opentelemetry/_constants.py	Changed the FIXED_PERCENTAGE_SAMPLER constant value from "microsoft.fixed.percentage" to "microsoft.fixed_percentage"
sdk/monitor/azure-monitor-opentelemetry/azure/monitor/opentelemetry/_utils/configurations.py	Added backward compatibility check to accept both old and new formats, and added null safety checks for sampler_arg
sdk/monitor/azure-monitor-opentelemetry/README.md	Updated documentation to reference the new format "microsoft.fixed_percentage"
sdk/monitor/azure-monitor-opentelemetry/samples/tracing/sampling_configurations.py	Updated sample code comment to use the new format "microsoft.fixed_percentage"
sdk/monitor/azure-monitor-opentelemetry/tests/utils/test_configurations.py	Added test case to verify backward compatibility with the old format "microsoft.fixed.percentage"