rust: compile gRPC bindings with Tonic (#4317)

Summary:
This patch switches our Rust protocol buffer bindings from `prost-build`
to `tonic-build`. This generates the same protobuf message types, since
Tonic interoperates with Prost, but can now also generate gRPC stubs.

Test Plan:
Check the doc server to see that our existing proto types still look
good, in the `rustboard_core::proto::tensorboard` package. We don’t have
gRPC services to generate yet, so our existing generated code hasn’t
changed. A follow-up commit will add a demo server as proof of concept.

wchargin-branch: rust-compile-grpc

tensorboard/data/server/BUILD

@@ -35,6 +35,7 @@ rust_library(
         "//third_party/rust:prost",
         "//third_party/rust:rand",
         "//third_party/rust:rand_chacha",
+        "//third_party/rust:tonic",
     ],
 )
 
@@ -89,7 +90,7 @@ rust_binary(
     edition = "2018",
     visibility = ["//visibility:private"],
     deps = [
-        "//third_party/rust:prost_build",
+        "//third_party/rust:tonic_build",
     ],
 )
 

tensorboard/data/server/gen_protos_tool.rs

@@ -26,9 +26,10 @@ fn main() -> std::io::Result<()> {
         dir.push("genproto");
         dir
     };
-    prost_build::Config::new()
+    tonic_build::configure()
         .out_dir(&out_dir)
-        .compile_protos(&["tensorboard/compat/proto/event.proto"], &["."])
+        .format(false) // don't run `rustfmt`; shouldn't be needed to build
+        .compile(&["tensorboard/compat/proto/event.proto"], &["."])
         .expect("compile_protos");
     Ok(())
 }