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
Summary
Closes #37
Checklist
fix
,enhancement
,documentation
,change
,breaking
,dependency
as they show up in the changelog