Store measurement values for all supported scales

[fniessink]

At the moment, the measurement value stored is the value for the current scale. If the user changes the scale, the old measurement values (as displayed in the graphs) are wrong. Solution: store the values for each scale.

[fniessink]

@lhelwerd This changes the way measurements are stored. Previously, each measurement document contained one "value" attribute. After this bug fix is released, each measurement document will from then on (history will not be changed) contain a value per scale that the metric supports. E.g:

{"count": {"value": "10", "status": "target_met"}, "percentage": {"value": "2", "status": "target_not_met"}, ...}