Document why XdsTestClient.check_channel_successful_calls is not using calls_succeeded

[sergiitk]

Context: grpc/grpc#35280 (comment)

Not sure why we didn't notice calls_succeeded in the first place.
I tried the oldest builds of xds test clients of each language, and all of them correctly set this field.

See #13 (comment).

cc @yashykt

[sergiitk]

In addition, there's no issue in outputting it too.

diff --git a/framework/rpc/grpc_channelz.py b/framework/rpc/grpc_channelz.py
index 59e1478..0972803 100644
--- a/framework/rpc/grpc_channelz.py
+++ b/framework/rpc/grpc_channelz.py
@@ -110,3 +110,4 @@ class ChannelzServiceClient(framework.rpc.grpc.GrpcClientHelper):
         result += (
-            f" call_started={channel.data.calls_started}"
+            f" calls_succeeded={channel.data.calls_succeeded}"
+            + f" calls_started={channel.data.calls_started}"
             + f" calls_failed={channel.data.calls_failed}"

I1211 16:37:10.451982 140704577574656 client_app.py:320] [psm-grpc-client-5cdf484df6-nd4x7] Detected successful calls to xDS control plane trafficdirector.googleapis.com:443, channel: <Channel channel_id=4 target=trafficdirector.googleapis.com:443 calls_succeeded=0 calls_started=2 calls_failed=0 state=READY>

[sergiitk]

OK, I remember now. Because it's a bidi stream, calls_succeeded won't be updated until the RPC is finished. Completely forgot about it. We should explain it better in a comment to check_channel_successful_calls.