[C++][FlightRPC] Flight fails building with protobuf v34.0+ due to functions updated with nodiscard

[raulcd]

Describe the bug, including details regarding any error messages, version, and platform.

I've seen this issue in the ARM64 macOS 14 GLib & Ruby job here:
https://github.com/apache/arrow/actions/runs/22618042997/job/65535748275?pr=49429

 [522/661] Building CXX object src/arrow/flight/CMakeFiles/arrow_flight_objlib.dir/transport/grpc/serialization_internal.cc.o
FAILED: [code=1] src/arrow/flight/CMakeFiles/arrow_flight_objlib.dir/transport/grpc/serialization_internal.cc.o 
/opt/homebrew/bin/ccache /usr/bin/c++ -DARROW_FLIGHT_EXPORTING -DARROW_HAVE_NEON -DARROW_WITH_TIMING_TESTS -DGRPC_ENABLE_ASYNC -DGRPC_NAMESPACE_FOR_TLS_CREDENTIALS_OPTIONS=grpc::experimental -DGRPC_USE_CERTIFICATE_VERIFIER -DGRPC_USE_TLS_CHANNEL_CREDENTIALS_OPTIONS -DPROTOBUF_USE_DLLS -I/Users/runner/work/arrow/arrow/build/cpp/src -I/Users/runner/work/arrow/arrow/cpp/src -I/Users/runner/work/arrow/arrow/cpp/src/generated -isystem /opt/homebrew/include -isystem /opt/homebrew/opt/openssl@3/include -fno-aligned-new -Qunused-arguments -fcolor-diagnostics -Wall -Wextra -Wdocumentation -DARROW_WARN_DOCUMENTATION -Wshorten-64-to-32 -Wno-missing-braces -Wno-unused-parameter -Wno-constant-logical-operand -Wno-return-stack-address -Wdate-time -Wno-unknown-warning-option -Wno-pass-failed -march=armv8-a -g -Werror -O0 -ggdb -g1 -std=c++20 -arch arm64 -fPIC -MD -MT src/arrow/flight/CMakeFiles/arrow_flight_objlib.dir/transport/grpc/serialization_internal.cc.o -MF src/arrow/flight/CMakeFiles/arrow_flight_objlib.dir/transport/grpc/serialization_internal.cc.o.d -o src/arrow/flight/CMakeFiles/arrow_flight_objlib.dir/transport/grpc/serialization_internal.cc.o -c /Users/runner/work/arrow/arrow/cpp/src/arrow/flight/transport/grpc/serialization_internal.cc
/Users/runner/work/arrow/arrow/cpp/src/arrow/flight/transport/grpc/serialization_internal.cc:356:9: error: ignoring return value of function declared with 'nodiscard' attribute [-Werror,-Wunused-result]
 356 | pb_stream.ReadString(&buffer, length);
 | ^~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~
1 error generated.
[523/661] Building CXX object src/arrow/flight/CMakeFiles/arrow_flight_objlib.dir/transport/grpc/grpc_server.cc.o
[524/661] Building CXX object src/arrow/flight/CMakeFiles/arrow_flight_objlib.dir/transport/grpc/grpc_client.cc.o
[525/661] Building CXX object src/arrow/flight/CMakeFiles/arrow_flight_objlib.dir/transport/grpc/protocol_grpc_internal.cc.o
ninja: build stopped: subcommand failed.

From what I can see protobuf has started to add a bunch of nodiscard to some of their methods which are part of v34.0:
protocolbuffers/protobuf@10af44b

Component(s)

C++, FlightRPC

[raulcd]

@tadeja can you comment here so I can assign the issue :)

[tadeja]

take