Checks on modifying metrics

[guilhermelowa]

Currently when we add / remove a metric, we update the file with the active metrics and update the header of the metrics.csv file (where the recordings are stored). I can see two problems with it: metrics with same name and adding a previously removed metric.

• When adding a metric with the same name as an active one, we have to make sure to include only one.
• When adding a metric previously removed, we shouldn't change the header on the metrics.csv file. Just keep writing to the respective column