grpc-java now only exposes the protoc plugin indirectly through a toolchain.
However, the current implementation of //bazel_tools.proto.bzl proto_gen
assumes direct access to the protoc plugin label.
https://github.com/grpc/grpc-java/commit/745aa0a2f570d74e7d7fe0c72334c24e21b8ab17
diff --git a/compiler/BUILD.bazel b/compiler/BUILD.bazel
index ef940741d..e50ecb66c 100644
--- a/compiler/BUILD.bazel
+++ b/compiler/BUILD.bazel
@@ -10,6 +10,7 @@ cc_binary(
     deps = [
         "@com_google_protobuf//:protoc_lib",
     ],
+    visibility = ["//visibility:public"],
 )
 
 java_library(