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

Checklist before review

• [ ] I added everything I wanted to add to this PR.
• [ ] [Code or tests only] I wrote/updated the necessary docstrings.
• [ ] [Code or tests only] I ran and passed tests locally.
• [ ] [Documentation only] I built the docs locally.
• [ ] My contribution is harmonious with the rest of the code: I'm not introducing repetitions.
• [ ] My code respects the adopted style, especially linting conventions.
• [ ] The title of this PR is explanatory on its own, enough to be understood as part of a changelog.
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