Add ability to include a build tag

This enables conditional compiling. This is particularly useful for
shared protobufs for common APIs, where not all consumers have the same
preferences around build size vs benefits of vtprotobuf. This allows the
consumer of the library to chose whether vtprotobuf is used, rather than
only the producer of the library.

Additionally, it provides guidance to use `vtprotobuf` as the tag. The
hope is that if multiple libraries do this, a user can just set that tag
instead of `-tags=envoy_vt,enable_vtprotobuf_prometheus,vtproto_k8s` or
something like that.

See envoyproxy/envoy#31172, where this will be
utilized (I think).

Makefile

@@ -64,6 +64,16 @@ gen-testproto: get-grpc-testproto gen-wkt-testproto install
 		testproto/proto2/scalars.proto \
 		testproto/unsafe/unsafe.proto \
 		|| exit 1;
+	$(PROTOBUF_ROOT)/src/protoc \
+		--proto_path=testproto \
+		--proto_path=include \
+		--go_out=. --plugin protoc-gen-go="${GOBIN}/protoc-gen-go" \
+		--go-vtproto_opt=paths=source_relative \
+		--go-vtproto_opt=buildTag=vtprotobuf \
+		--go-vtproto_out=allow-empty=true:./testproto/buildtag --plugin protoc-gen-go-vtproto="${GOBIN}/protoc-gen-go-vtproto" \
+		-I$(PROTOBUF_ROOT)/src \
+		testproto/empty/empty.proto \
+		|| exit 1;
 
 get-grpc-testproto: install
 	$(PROTOBUF_ROOT)/src/protoc \

README.md

@@ -108,10 +108,20 @@ The following features can be generated:
             --go-vtproto_opt=pool=vitess.io/vitess/go/vt/proto/query.Row \
             --go-vtproto_opt=pool=vitess.io/vitess/go/vt/proto/binlogdata.VStreamRowsResponse \
     ```
+6. (Optional) if you want to selectively compile the generate `vtprotobuf` files, the `--vtproto_opt=buildTag=<tag>` can be used.
 
-6. Compile the `.proto` files in your project. You should see `_vtproto.pb.go` files next to the `.pb.go` and `_grpc.pb.go` files that were already being generated.
+    When using this option, the generated code will only be compiled in if a build tag is provided.
 
-7. (Optional) Switch your RPC framework to use the optimized helpers (see following sections)
+    It is recommended, but not required, to use `vtprotobuf` as the build tag if this is desired, especially if your project is imported by others.
+    This will reduce the number of build tags a user will need to configure if they are importing multiple libraries following this pattern.
+
+    When using this option, it is strongly recommended to make your code compile with and without the build tag.
+    This can be done with type assertions before using `vtprotobuf` generated methods.
+    The `grpc.Codec{}` object (discussed below) shows an example.
+
+7. Compile the `.proto` files in your project. You should see `_vtproto.pb.go` files next to the `.pb.go` and `_grpc.pb.go` files that were already being generated.
+
+8. (Optional) Switch your RPC framework to use the optimized helpers (see following sections)
 
 ## `vtprotobuf` package and well-known types
 

cmd/protoc-gen-go-vtproto/main.go

@@ -26,6 +26,7 @@ func main() {
 	f.Var(&cfg.Poolable, "pool", "use memory pooling for this object")
 	f.BoolVar(&cfg.Wrap, "wrap", false, "generate wrapper types")
 	f.StringVar(&features, "features", "all", "list of features to generate (separated by '+')")
+	f.StringVar(&cfg.BuildTag, "buildTag", "", "the go:build tag to set on generated files")
 
 	protogen.Options{ParamFunc: f.Set}.Run(func(plugin *protogen.Plugin) error {
 		gen, err := generator.NewGenerator(plugin, strings.Split(features, "+"), &cfg)

generator/generator.go

@@ -37,9 +37,10 @@ func (o ObjectSet) Set(s string) error {
 }
 
 type Config struct {
-	Poolable       ObjectSet
-	Wrap           bool
-	AllowEmpty     bool
+	Poolable   ObjectSet
+	Wrap       bool
+	AllowEmpty bool
+	BuildTag   string
 }
 
 type Generator struct {
@@ -97,6 +98,11 @@ func (gen *Generator) generateFile(gf *protogen.GeneratedFile, file *protogen.Fi
 		LocalPackages: gen.local,
 	}
 
+	if p.Config.BuildTag != "" {
+		// Support both forms of tags for maximum compatibility
+		p.P("//go:build ", p.Config.BuildTag)
+		p.P("// +build ", p.Config.BuildTag)
+	}
 	p.P("// Code generated by protoc-gen-go-vtproto. DO NOT EDIT.")
 	if bi, ok := debug.ReadBuildInfo(); ok {
 		p.P("// protoc-gen-go-vtproto version: ", bi.Main.Version)