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>

docs/sources/_index.md

@@ -91,9 +91,9 @@ aliases = ["/docs/grafana/v1.1", "/docs/grafana/latest/guides/reference/admin",
       <img src="/img/docs/logos/icon_prometheus.svg" >
       <h5>Prometheus</h5>
     </a>
-    <a href="{{< relref "features/datasources/stackdriver.md" >}}" class="nav-cards__item nav-cards__item--ds">
-      <img src="/img/docs/logos/icon_stackdriver.svg">
-      <h5>Google Stackdriver</h5>
+    <a href="{{< relref "features/datasources/cloudmonitoring.md" >}}" class="nav-cards__item nav-cards__item--ds">
+      <img src="/img/docs/logos/icon_cloudmonitoring.svg">
+      <h5>Google Cloud Monitoring</h5>
     </a>
     <a href="{{< relref "features/datasources/cloudwatch.md" >}}" class="nav-cards__item nav-cards__item--ds">
       <img src="/img/docs/logos/icon_cloudwatch.svg">