content/integrations/bigtable: provide a screenshot when the GCP client library is properly instrumented

[odeke-em]

This is a TODO for @songy23(thank you) from https://github.com/census-instrumentation/opencensus-website/pull/208#pullrequestreview-142475419 and particularly https://github.com/census-instrumentation/opencensus-website/pull/208#discussion_r206712643 where despite the app level spans showing up in the trace, the Google Cloud Bigtable traces and annotations didn't show up.

This perhaps shows that maybe their traces aren't being properly ended. The Bigtable GCP library was already instrumented in September 2017 in https://github.com/GoogleCloudPlatform/cloud-bigtable-client/pull/1506 and a few hours ago @bogdandrutu fixed some issues that their library has https://github.com/GoogleCloudPlatform/cloud-bigtable-client/pull/1901 so we'll need to keep vigilant for when they release a new version of their client on Maven Central.