Gax has introduced FlowControl settings in [1] (see setFlowControlSettings). This makes it seem like Flow Control is available in the APIs that support BatchSettings, such as Pub/Sub's Publisher [2]. There are two issues here:

1. FlowControlSettings settings in GAX have no way of communicating to the user whether a particular client implements them. This is actually true of all batching settings. This is a recipe for confusion.
2. In the particular case of Pub/Sub client, publisher flow control is a desirable feature in itself. We either need to decide to implement it and turn this bug into a feature request and offer explicit failure and documentation if we decided not to.

[1] http://googleapis.github.io/gax-java/1.4.1/apidocs/com/google/api/gax/batching/BatchingSettings.Builder.html#setFlowControlSettings-com.google.api.gax.batching.FlowControlSettings-
[2] https://github.com/googleapis/google-cloud-java/blob/master/google-cloud-clients/google-cloud-pubsub/src/main/java/com/google/cloud/pubsub/v1/Publisher.java#L124
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