We have two reports where users are hitting system limits when creating large numbers of GAPIC-based clients:

• PubSub: Errno 24: too many open files with multiple publishers google-cloud-python#5523
• Speech: Memory leak when creating SpeechClient instance for each request google-cloud-python#6077

While that is not an expected usage pattern, we should still expose a way for callers to perform an explicit shutdown on the transport / channel in order to free up resources in such cases.