Inside ManagedHttpJsonChannel's awaitTermination() method,
https://github.com/googleapis/gapic-generator-java/blob/4054bc668daf797a8fce798843e0ec4cda669faa/gax-java/gax-httpjson/src/main/java/com/google/api/gax/httpjson/ManagedHttpJsonChannel.java#L105-L108

there is a TODO that simply returns false. Implement this functionality.