Metric definitions will start recording data when they are created, they will never pull past data by default.
You can generate past data by writing a script (Scheduled Script or Fix Script) that will retroactively create and populate metric_instance records. To capture the right values for "Start", "End", "Field value", etc., you can use the sys_audit table.
Basically you would iterate through the records for which you want to create historic data and then for each record, you would look up the relevant sys_audit records and create a metric_instance record with the correct values.
It might be best from a performance perspective to do this in a Schedule Script execution which runs regularly, does a couple hundred or thousand records at a time, and somehow marks after each run where it's at (maybe going in order of "sys_created_on" and recording the last processed record's "sys_created_on" in a sys_property.
