fix(handlers): always add the migration_path label to HTTP metrics

When migration.enabled was false, the migration_path label would not be added to the HTTP metrics, but Prometheus expects it to be always present (either set or not). This was breaking the metrics emission.

This change makes sure that the label is always present. When migration.enabled is false, the label is set to an empty string. When migration.enabled is true, the label is set to either old or new.

Related to #391 (closed).