Add battery charge metric

[ccremer]

Summary

• Adds battery charge percentage metric (ranging from 0 to 1)

Closes #37

Checklist

• [x] Keep pull requests small so they can be easily reviewed.
• [x] Categorize the PR by setting a good title and adding one of the labels: fix, enhancement, documentation, change, breaking, dependency as they show up in the changelog
• [x] Update documentation.
• [x] Update tests.
• [x] Link this PR to related issues.