Adds instructions to access Grafana in GKE tutorial

[jlerche]

What problem does this PR solve?

#404

What is changed and how it works?

Adds instruction in the GKE tutorial to access Grafana dashboard

Check List

Tests

• No code

Code changes

• Has documents change

Side effects

Related changes

[CLAassistant]

All committers have signed the CLA.

[gregwebs]

LGTM

[tennix]

I think it's better to use a load balancer for the tutorial, so new users can easily access it via a public DNS name.
Adding extra value: monitor.grafana.service.type=LoadBalancer when installing TiDB cluster.

helm install ./charts/tidb-cluster -n demo --namespace=tidb --set pd.storageClassName=pd-ssd,tikv.storageClassName=pd-ssd,monitor.grafana.service.type=LoadBalancer

And then get grafana service external IP: kubectl get svc -n tidb demo-grafana

[gregwebs]

@tennix that will create additional security concerns that I think we should avoid getting into during the tutorial. But your comment would make sense to add to the user guide.

[gregwebs]

We can merge this now and keep improving it as needed.