This aspect of gathering data for metrics involves two components: That the data can actually be gathered and that gathering the data is relatively easy for team members.
Sometimes data cannot be gathered. Either the data is so obscure or no measuring instrument for that data set exists. Other times, the data is indiscernible, incomplete, or otherwise problematic to collect. Data like this will not be reliable or reportable.
Sometimes team members are unable to gather data. If you are trying to increase sales of video games, the precise names and addresses of owners of specific games is not a feasible metric to follow. Team members just won’t have the capability to follow this. Data should be easy for team members to collect so it will be more likely to be reported.