[Tests] Add unit tests for isValidMetricName validator

[gonzaloriestra]

What page / article is this about?

This PR adds unit tests for isValidMetricName in @shopify/cli-kit.

What changed?

• Added unit test file packages/cli-kit/src/public/node/vendor/otel-js/utils/validators.test.ts.
• Tested isValidMetricName with valid metric names (valid_metric_name, metric, _metric, METRIC_NAME) returning true.
• Tested isValidMetricName with invalid metric names (invalid-metric, 123metric, metric.name, metric@name, metric name) returning false and triggering a diagnostic warning via diag.warn.

How to test your changes?

CI

Checklist

• I have read the Contributing Guidelines.
• I have signed the Contributor License Agreement.
• I have added tests to cover my changes.
• I have updated the documentation accordingly.

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PR created automatically by Jules for task 15547795319124221370 started by @gonzaloriestra

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