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/converter/prom.go

@@ -1140,8 +1140,8 @@ streamField:
 	return parsedLabelsMap, structuredMetadataMap, nil
 }
 
-func resultTypeToCustomMeta(resultType string) map[string]string {
-	return map[string]string{"resultType": resultType}
+func resultTypeToCustomMeta(resultType string) map[string]any {
+	return map[string]any{"resultType": resultType}
 }
 
 func timeFromFloat(fv float64) time.Time {