Extract construction of platform attributes in telemetry/decorators.py.
Extract construction of platform attributes in telemetry/decorators.py.

This is a refactoring change.

Review URL: https://codereview.chromium.org/1528143003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#366149}

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