I've also tried:
# Generated Proto files
file(GLOB_RECURSE proto_files "${CMAKE_CURRENT_SOURCE_DIR}/*.proto")
PROTOBUF_GENERATE_CPP(proto_srcs proto_hdrs ${proto_files} PLUGIN protoc-gen-grpc=${GRPC_PLUGIN})
add_custom_target(proto_dep DEPENDS ${proto_srcs} ${proto_hdrs})
message(STATUS "Generated source files are ${proto_srcs}")
However, I do not see any .grpc.pb.cc files or know how to get the list of them to add to compilation of my library with add_library. If I understand the docs correctly, I have to compile the *.pb.cc files as well as the *.grpc.pb.cc files.