Prometheus: Fix interpolating minStep for range queries when incremental querying enabled (#107715)

* return calculatedMinStep in first frame of the response

* update test files

* make the custom metadata as type of map[string]any

* append calculatedMinStep to custom metadata

* fix exemplar frame checking

pkg/promlib/models/result.go

@@ -20,7 +20,7 @@ func ResultTypeFromFrame(frame *data.Frame) ResultType {
 	if frame.Meta.Custom == nil {
 		return ResultTypeUnknown
 	}
-	custom, ok := frame.Meta.Custom.(map[string]string)
+	custom, ok := frame.Meta.Custom.(map[string]any)
 	if !ok {
 		return ResultTypeUnknown
 	}