Prometheus: Min step defaults to seconds when no unit is set to prevent errors when running alerts. (#30966)

* update interval to fix no unit

* fix bug

* update with new method

* format go

* add test

* change test method to add test error nil

* add simplejson and model package

* change json format

* add parentheses

* add simplejson to function

* change mode to model

* move assert import package

* add one enter to avoid goimport error

* change to test case