Fix naming of metrics, clarify PULSE RATE vs HEART RATE

[smoia]

Closes #

Proposed Changes

• Changes name of heart_rate_variability to heart_rate, add heart_rate_variability

Change Type

• bugfix (+0.0.1)
• minor (+0.1.0)
• major (+1.0.0)
• refactoring (no version update)
• test (no version update)
• infrastructure (no version update)
• documentation (no version update)
• other

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• I added everything I wanted to add to this PR.
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