RatesAnalysis: Save metadata for online rates

Follow up to !55100 (merged)

Save standard rates&cost metadata file with run details. Also, retrieve the avg lumi, needed for comparison on the website.

If more than one set of keys is found for the passed run range, the first found will be used, and a warning will be printed. In the future, the script will handle changing prescale sets.

Tagging @tamartin