Update monitoring diagnostics provenance record for new monitoring HTML output

[bsolino]

Is your feature request related to a problem? Please describe.
I am currently working on improving the ESMValTool HTML output to add new features like filtering plots by e.g. realm, variable or plot type.
To be able to do this, this information has to be available on the provenance record. Currently the documentation doesn't mention how to add variables. I have some ideas (e.g., under "long_names", see the linked PR to see the latest implementation), but I'm interested in having feedback on how to add them.