You have created a metric definition and now you want the metric instances to be created for the existing records.
Unfortunately, there is no such way/mechanism to create metric instances for already existing records.
When inserting or updating a record in the Task table hierarchy (e.g. incident), one of the Business Rules that runs with After engines is "metrics events".
These events are processed using Metric Update Script Action, which uses MetricInstance Script Include, and as a result, if a relevant metric_definition is found, metric_instance records are created.
This is how metric instances are created.