...
A "metric" is just a type of study associated with the subject, set in the subjects GUI. A separate very simple table records all covariate_values in that metric.
Table metric
| 'study_id' | 'int(11)' | 'NO' |
| 'covariate_value_id' | 'int(11)' | 'NO' |
Note that one metric could have repeated entries - this might be useful to make up for missing data (I'm not sure). And of course one metric can be used in multiple project studies.