docs: add note about directExecutor in example tests.

[rubensayshi]

Since the HelloWorld tests are atm the "documentation" for how to do unit tests for grpc I think it would be good to add a note about the danger of the directExecutor(), particularly in bidirectional streaming.

I considered removing the directExecutor but it seems a common pattern in the grpc test suite and has it's merits so warning devs seems like a better solution.

related:

• Potential deadlock due to calling callbacks while holding a lock #3084
• Update examples to use Executor #2263

[ejona86]

We use a direct executor so the test is deterministic.

#2263 is unrelated; it's about using a fixed-size executor instead of our default cached thread pool executor, which can grow to any size. #3084 is a bug in our implementation, but it also requires a second thread to be in play. It looks like any streaming is likely impacted. I didn't realize #3084 was still open.

For now, maybe something along these lines:

directExecutor() makes it easier to have deterministic tests. However, if your implementation uses another thread and uses streaming it is better to use the default executor, to avoid hitting bug #3084 .

[rubensayshi]

@ejona86 okay, updated with your suggestion.

[ejona86]

@zhangkun83, could you review as well?

[ejona86]

@rubensayshi, thank you!