docs(samples): Add code sample for optimistic subscribe

[mukund-ananthu]

Related documentation: https://cloud.google.com/pubsub/docs/troubleshooting#using_excessive_administrative_operations

Fixes #1183 🦕

[snippet-bot]

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• samples/snippets/subscriber.py:107, tag pubsub_optimistic_subscribe

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[mukund-ananthu]

@kamalaboulhosn Gentle bump. PTAL when free.

[kamalaboulhosn]

Should we instead say "Successfully subscribed until the timeout passed."? As it is, it sounds like this is an error state.

[mukund-ananthu]

Sure, I've replaced the statement with the suggested statement.

[kamalaboulhosn]

Should we do something if this test fails?

[mukund-ananthu]

If the call to create_subscription() fails, then it would throw an exception and the control would not arrive at the if subscription check.

python-pubsub/google/pubsub_v1/services/subscriber/client.py

Lines 743 to 744 in 5094605

	If the subscription already exists, returns ``ALREADY_EXISTS``.
	If the corresponding topic doesn't exist, returns ``NOT_FOUND``.

The check should always evaluate to true and was added as a precautionary null check. I've now additionally included an else clause that raises a ValueError in case this check fails.

I've now also enclosed the create subscription code in a try / except clause to handle any exceptions thrown by the create_subscription() call. Reasons for not doing this earlier is to keep the sample code simple, similar to the pubsub_create_pull_subscription() sample code in the file:

python-pubsub/samples/snippets/subscriber.py

Lines 86 to 94 in 5094605

	# Wrap the subscriber in a 'with' block to automatically call close() to
	# close the underlying gRPC channel when done.
	with subscriber:
	subscription = subscriber.create_subscription(
	request={"name": subscription_path, "topic": topic_path}
	)
	print(f"Subscription created: {subscription}")
	# [END pubsub_create_pull_subscription]

[mukund-ananthu]

@kamalaboulhosn Addressed comments. PTAL when free.