Comment 5 for bug 2013187

I've received permission to share this.

Original methodology from Erlon Cruz: https://github.com/gnocchixyz/gnocchi/issues/1050#issuecomment-542753802

Credit to Nikita Koltsov for the initial rework to use temporary tables and stored procedures to speed up some of the code.

I built upon Nikita's code and added additional cleanup/reworks/optimizations to bring the code to where it is now.

The code is not perfect, and there's some low-hanging optimization regarding volume/image metric cleanup which I've hinted at in one of the comments in the code, but which I simply have not gotten around to. Nonetheless, submitting as-is in case it can help give a good starting point for something baked into the charm..