Only way is to make it self documenting, when a metric is set up, when you are navigating the graphs etc there should be some text defining where this data came

Only way is to make it self documenting, when a metric is set up, when you are navigating the graphs etc there should be some text defining where this data came from. It also depends if lack of data means something to your system. In my system a lack of data for a metric for a time span x implies that something is wrong and should be alerted. In some systems this might not matter. This provides a self fulfilling habit of removing dead metrics