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/services/hardcoded_metrics.go

@@ -0,0 +1,43 @@
+package services
+
+import (
+	"fmt"
+
+	"github.com/grafana/grafana/pkg/tsdb/cloudwatch/constants"
+	"github.com/grafana/grafana/pkg/tsdb/cloudwatch/models"
+)
+
+var GetHardCodedDimensionKeysByNamespace = func(namespace string) ([]string, error) {
+	var dimensionKeys []string
+	exists := false
+	if dimensionKeys, exists = constants.NamespaceDimensionKeysMap[namespace]; !exists {
+		return nil, fmt.Errorf("unable to find dimensions for namespace '%q'", namespace)
+	}
+	return dimensionKeys, nil
+}
+
+var GetHardCodedMetricsByNamespace = func(namespace string) ([]models.Metric, error) {
+	response := []models.Metric{}
+	exists := false
+	var metrics []string
+	if metrics, exists = constants.NamespaceMetricsMap[namespace]; !exists {
+		return nil, fmt.Errorf("unable to find metrics for namespace '%q'", namespace)
+	}
+
+	for _, metric := range metrics {
+		response = append(response, models.Metric{Namespace: namespace, Name: metric})
+	}
+
+	return response, nil
+}
+
+var GetAllHardCodedMetrics = func() []models.Metric {
+	response := []models.Metric{}
+	for namespace, metrics := range constants.NamespaceMetricsMap {
+		for _, metric := range metrics {
+			response = append(response, models.Metric{Namespace: namespace, Name: metric})
+		}
+	}
+
+	return response
+}