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/dimension_keys_request.go

@@ -2,8 +2,6 @@ package request
 
 import (
 	"net/url"
-
-	"github.com/grafana/grafana/pkg/tsdb/cloudwatch/constants"
 )
 
 type DimensionKeysRequestType uint32
@@ -22,7 +20,7 @@ type DimensionKeysRequest struct {
 }
 
 func (q *DimensionKeysRequest) Type() DimensionKeysRequestType {
-	if _, exist := constants.NamespaceMetricsMap[q.Namespace]; !exist {
+	if isCustomNamespace(q.Namespace) {
 		return CustomMetricDimensionKeysRequest
 	}