Cloudwatch: Refactor dimension values resource request (#57346)

* refactor dimension values backend

* fix test file name

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

@@ -0,0 +1,16 @@
+package request
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestResourceRequest(t *testing.T) {
+	t.Run("Should return an error if region is not provided", func(t *testing.T) {
+		request, err := GetDimensionValuesRequest(map[string][]string{})
+		require.Nil(t, request)
+		assert.Equal(t, "region is required", err.Error())
+	})
+}