refactor: move system cpu-related metrics to cpu namespace

[rogercoll]

Partially fixes #1873 (renaming)

Changes

Current system.cpu.* metrics do not reflect the CPU time/usage over the whole system but relative to a logical CPU. Each reported metric is specific to a logical CPU (system.cpu.logical_number). This PR proposes moving them to a new CPU namespace that will define metrics relative to a unique logical CPU. Additionally, the system.cpu.logical_number has been renamed to cpu.logical_number as it only applies to cpu namespace metrics.

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• schema-next.yaml updated with changes to existing conventions.