Cloudwatch: Refactor metrics resource request (#57424)

* refactor metrics request

* Update pkg/tsdb/cloudwatch/routes/dimension_keys_test.go

Co-authored-by: Shirley <4163034+fridgepoet@users.noreply.github.com>

* return metric struct value intead of pointer

* make it possible to test hard coded metrics service

* test all paths in route

* fix broken test

* fix one more broken test

* add integration test

Co-authored-by: Shirley <4163034+fridgepoet@users.noreply.github.com>

pkg/tsdb/cloudwatch/models/request/utils.go

@@ -3,6 +3,8 @@ package request
 import (
 	"encoding/json"
 	"fmt"
+
+	"github.com/grafana/grafana/pkg/tsdb/cloudwatch/constants"
 )
 
 func parseDimensionFilter(dimensionFilter string) ([]*Dimension, error) {
@@ -42,3 +44,10 @@ func parseDimensionFilter(dimensionFilter string) ([]*Dimension, error) {
 
 	return dimensions, nil
 }
+
+func isCustomNamespace(namespace string) bool {
+	if _, ok := constants.NamespaceMetricsMap[namespace]; ok {
+		return false
+	}
+	return true
+}