Stackdriver: Rename Stackdriver to Google Cloud Monitoring (#25807)

* Update backend

* Update frontend

* Keep old plugin id

* Update docs

* Place doc images to a new directory

* Legacy support for stackdriver-auto alignment

* Consistent plugin name

* Apply suggestions from code review

Co-authored-by: Marcus Efraimsson <marcus.efraimsson@gmail.com>
Co-authored-by: Tobias Skarhed <1438972+tskarhed@users.noreply.github.com>

* Update docs

* Update public/app/plugins/datasource/cloud-monitoring/README.md

Co-authored-by: Marcus Efraimsson <marcus.efraimsson@gmail.com>

* Add reference to the data source formerly being named Stackdriver

* Update pkg/models/datasource.go

Co-authored-by: Carl Bergquist <carl@grafana.com>

* Fix gofmt

Co-authored-by: Marcus Efraimsson <marcus.efraimsson@gmail.com>
Co-authored-by: Tobias Skarhed <1438972+tskarhed@users.noreply.github.com>
Co-authored-by: Carl Bergquist <carl@grafana.com>

pkg/cmd/grafana-server/main.go

@@ -20,6 +20,7 @@ import (
 	_ "github.com/grafana/grafana/pkg/services/alerting/notifiers"
 	"github.com/grafana/grafana/pkg/setting"
 	_ "github.com/grafana/grafana/pkg/tsdb/azuremonitor"
+	_ "github.com/grafana/grafana/pkg/tsdb/cloudmonitoring"
 	_ "github.com/grafana/grafana/pkg/tsdb/cloudwatch"
 	_ "github.com/grafana/grafana/pkg/tsdb/elasticsearch"
 	_ "github.com/grafana/grafana/pkg/tsdb/graphite"
@@ -28,7 +29,6 @@ import (
 	_ "github.com/grafana/grafana/pkg/tsdb/opentsdb"
 	_ "github.com/grafana/grafana/pkg/tsdb/postgres"
 	_ "github.com/grafana/grafana/pkg/tsdb/prometheus"
-	_ "github.com/grafana/grafana/pkg/tsdb/stackdriver"
 	_ "github.com/grafana/grafana/pkg/tsdb/testdatasource"
 )