GCP CSM Observability Java client use XdsCredentials

[kannanjgithub]

To demonstrate using the CSM mesh client to talk to cloud run services in user guide, introduce a command line option to use XdsCredentials.

[kannanjgithub]

It seems weird to do this in the observability example. Cloud Run is unrelated to csm observability.

gcp CSM Observability examples are already in gRPC.io docker hub. I just tried avoiding creating and maintaining another set of docker images without exposing metrics on a Prometheus port. In the user guide we pull the example code from docker.

[kannanjgithub]

Also the Proxyless service-mesh guide already uses the CSM Observability example as well: https://cloud.google.com/service-mesh/docs/gateway/proxyless-grpc-mesh image: grpc/csm-o11y-example-cpp-server:latest

[ejona86]

@kannanjgithub, where did this end up going?