Improve the typing of the example in BaseApiClient

The `mypy-protobuf` plugin generates a `XxxAsyncStub` class for each
service, which have proper type information to be used with async. This
still needs some hacks (type ignore) to make it work, but it's better
than using the normal stubs and having to cast on each call.

Signed-off-by: Leandro Lucarella <luca-frequenz@llucax.com>

Author: llucax

src/frequenz/client/base/client.py

@@ -42,7 +42,9 @@ class BaseApiClient(abc.ABC, Generic[StubT]):
         from frequenz.client.base.streaming import GrpcStreamBroadcaster
         from frequenz.channels import Receiver
 
-        # These classes are normally generated by protoc
+        # These classes are normally generated by protoc, please use mypy-protobuf to
+        # get proper type checking and the XxxAsyncStub class (only for typing)
+        # generated.
         class ExampleRequest:
             int_value: int
             str_value: str
@@ -51,6 +53,9 @@ class ExampleResponse:
             float_value: float
 
         class ExampleStub:
+            ...
+
+        class ExampleAsyncStub:
             async def example_method(
                 self,
                 request: ExampleRequest  # pylint: disable=unused-argument
@@ -65,10 +70,14 @@ class ExampleResponseWrapper:
             def __init__(self, response: ExampleResponse):
                 self.transformed_value = f"{response.float_value:.2f}"
 
-        class MyApiClient(BaseApiClient[ExampleStub]):
+        class MyApiClient(BaseApiClient[ExampleAsyncStub]):
             def __init__(self, server_url: str, *, connect: bool = True):
                 super().__init__(
-                    server_url, ExampleStub, connect=connect
+                    server_url,
+                    # We need to type ignore here because the generated normal and
+                    # async stubs are not type-compatible with each other.
+                    ExampleStub,  # type: ignore[arg-type]
+                    connect=connect
                 )
                 self._broadcaster = GrpcStreamBroadcaster(
                     "stream",