Pub/Sub: add SubscriberClient.close() to examples

[anguillanneuf]

New PR based on @pradn's #2959.

Close subscriber clients after use either explicitly by calling the close() method or implicitly by wrapping them in a context manager.

[gguuss]

LGTM with SME approval from @hongalex or @feywind, tests are passing and style looks good. I noticed you're using different patterns in some snippets:

subscriber = pubsub_v1.SubScriberClient()

...

subscriber.close()

versus

subscriber = pubsub_v1.SubscriberClient()

with subscriber:
    try:
        streaming_pull_future.result(timeout=timeout)
    except:
        streaming_pull_future.cancel()

And just want to make sure you're intentionally doing this (it looks like the first pattern you're using in synchronous operations and the second for streaming).

[anguillanneuf]

Thanks for reviewing @gguuss! That is intended. They are interchangeable.

[feywind]

I admit I don't know much about the Python client libraries, but nothing looks scary in there.