integrations/googlecloud/go: add pubsub

[odeke-em]

While examining @jadekler's filed issue https://github.com/census-ecosystem/opencensus-go-exporter-stackdriver/issues/41 I noticed that in his repro https://gist.github.com/jadekler/5636acc62eb0048cad15206ef5587650 he is trying to collect stats from pubsub and noticed that googlecloud's pubsub go package is instrumented with OpenCensus at least for metrics https://github.com/GoogleCloudPlatform/google-cloud-go/blob/e14b7b4aa815fab3cebdd0cc0462f94aed282791/pubsub/go18.go

We need to add information about this instrumentation on the OpenCensus website and provide an end-to-end runnable example that anyone can use and inspect stats.