Is your feature request related to a problem? Please describe.
Creating a large amount of JsonStreamWriters will create a large amount of threads named gax-xxx, resulting in poor application performance and increased write latency. (Thread blocking will occur frequently.)

Describe the solution you'd like
We need a way to control the creation of gax threads.
For this purpose, we would like to have a mechanism to accept an arbitrary ExecutorProvider in JsonStreamWriter.Builder.

Additional context
Although gax threads are used within java-gax for things like retry processing, we only use the default stream and do not need a large amount of gax threads.
We have actually created a custom library implementing the solution described above and have confirmed that it works well.