GH-34047: [C++][FlightRPC] Make DoAction warning less prominent (#34182)

### Rationale for this change

The warning added for when you fail to consume a result stream (and get the final error/success result) is a bit aggressive. Ignore it in most cases.

### What changes are included in this PR?

Drop the warning to DEBUG level and don't log if it's just a cancellation.

### Are these changes tested?

Existing tests cover it.

### Are there any user-facing changes?

Users should see less inactionable warnings.
* Closes: #34047

Authored-by: David Li <li.davidm96@gmail.com>
Signed-off-by: David Li <li.davidm96@gmail.com>

Author: lidavidm

cpp/src/arrow/flight/transport/grpc/grpc_client.cc

@@ -510,8 +510,12 @@ class GrpcResultStream : public ResultStream {
   ~GrpcResultStream() override {
     if (stream_) {
       rpc_.context.TryCancel();
-      ARROW_WARN_NOT_OK(FromGrpcStatus(stream_->Finish(), &rpc_.context),
-                        "DoAction result was not fully consumed");
+      auto status = FromGrpcStatus(stream_->Finish(), &rpc_.context);
+      if (!status.ok() && !status.IsCancelled()) {
+        ARROW_LOG(DEBUG)
+            << "DoAction result was not fully consumed, server returned error: "
+            << status.ToString();
+      }
     }
   }