dev: LogLevel Warn -> Debug on AttachStream has been cancelled (#443)

Author: KirillKurdyukov

src/Ydb.Sdk/CHANGELOG.md

@@ -1,3 +1,5 @@
+- dev: LogLevel `Warning` -> `Debug` on AttachStream has been cancelled.
+
 ## v0.18.0
 
 - Disable Discovery mode: skip discovery step and client balancing and use connection to start endpoint ([#420](https://github.com/ydb-platform/ydb-dotnet-sdk/issues/420)).

src/Ydb.Sdk/src/Services/Query/SessionPool.cs

@@ -97,6 +97,13 @@ protected override async Task<Session> CreateSession()
                 }
                 catch (Driver.TransportException e)
                 {
+                    if (e.Status.StatusCode == StatusCode.Cancelled)
+                    {
+                        Logger.LogDebug("AttachStream is cancelled (possible grpcChannel is closing)");
+
+                        return;
+                    }
+
                     Logger.LogWarning(e, "Session[{SessionId}] is deactivated by transport error", sessionId);
                 }
             }