CloudMonitor: Correctly encode default project response (#49510) (#49611)

* Correctly encode default project response

* Make getGCEDefaultProject a method of Service and add test

* Handle error appropriately

* Update test and function definition

(cherry picked from commit ccb8888055)

Co-authored-by: Andreas Christou <andreas.christou@grafana.com>

pkg/tsdb/cloudmonitoring/resource_handler_test.go

@@ -1,7 +1,9 @@
 package cloudmonitoring
 
 import (
+	"context"
 	"encoding/json"
+	"fmt"
 	"io/ioutil"
 	"net/http"
 	"net/http/httptest"
@@ -9,6 +11,7 @@ import (
 
 	"github.com/grafana/grafana-plugin-sdk-go/backend"
 	"github.com/grafana/grafana-plugin-sdk-go/backend/instancemgmt"
+	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 )
 
@@ -285,3 +288,23 @@ func Test_processData_functions(t *testing.T) {
 		})
 	}
 }
+
+func Test_getGCEDefaultProject(t *testing.T) {
+	project := "test-project"
+	s := Service{
+		im: &fakeInstance{
+			services: map[string]datasourceService{
+				cloudMonitor: {
+					url:    routes[cloudMonitor].url,
+					client: &http.Client{},
+				},
+			},
+		},
+		gceDefaultProjectGetter: func(ctx context.Context) (string, error) {
+			return project, nil
+		},
+	}
+
+	assert.HTTPSuccess(t, s.getGCEDefaultProject, "GET", "/gceDefaultProject", nil)
+	assert.HTTPBodyContains(t, s.getGCEDefaultProject, "GET", "/gceDefaultProject", nil, fmt.Sprintf("\"%v\"", project))
+}