docs(grpc): warning for multiple threads (#3747)

* feat: raise error when GRPCClient is raised outside the main thread

* docs: showcase grpc client threading limitation

* docs: add note

* chore: apply suggestions

* test: add grpc client threading

* chore: reformat error message

* chore: apply suggestions

* chore: apply suggestions

* docs(grpc): warning for multiple threads

* revert(grpc): raise if non-main thread

Co-authored-by: Deepankar Mahapatro <deepankar.mahapatro@jina.ai>

Author: alaeddine-13

docs/fundamentals/flow/flow-as-a-service.md

@@ -102,6 +102,13 @@ c.post('/')
 GRPCClient@14744[S]:connected to the gateway at 0.0.0.0:12345!
 ```
 
+
+````{warning}
+
+Multiple gRPC Client cannot be spawned using `Threads` because of an [upstream issue](https://github.com/grpc/grpc/issues/25364). Use `multiprocessing` instead.
+````
+
+
 ### via WebSocket
 
 ```python

tests/unit/test_gateway.py

@@ -22,7 +22,7 @@ def test_compression(compress_algo):
 
 
 @pytest.mark.slow
-@pytest.mark.parametrize('protocol', ['websocket', 'grpc', 'http'])
+@pytest.mark.parametrize('protocol', ['websocket', 'http'])
 def test_gateway_concurrency(protocol, reraise):
     PORT_EXPOSE = 12345
     CONCURRENCY = 2