Add helium_level to the list of smartmon attributes to report.

[benlivengood]

Some of my hard drives export 22 Helium_Levelas an attribute, and I want to monitor them.

I've tested this change locally to make sure the new metrics appear in prometheus.

[dswarbrick]

Thanks, but since the smartmon.sh script is essentially superseded by the smartmon.py, and will likely be removed at some point in future, can you please add this to smartmon.py also?