protoc-gen-go/grpc: pass file descriptor for service as Metadata

Bump support package version number from 2 to 3.

protoc-gen-go/grpc/grpc.go

diff --git a/protoc-gen-go/grpc/grpc.go b/protoc-gen-go/grpc/grpc.go
index 291cbdb..a29a8d2 100644
--- a/protoc-gen-go/grpc/grpc.go
+++ b/protoc-gen-go/grpc/grpc.go
@@ -48,7 +48,7 @@
 // It is incremented whenever an incompatibility between the generated code and
 // the grpc package is introduced; the generated code references
 // a constant, grpc.SupportPackageIsVersionN (where N is generatedCodeVersion).
-const generatedCodeVersion = 2
+const generatedCodeVersion = 3
 
 // Paths for packages used by code generated in this file,
 // relative to the import_prefix of the generator.Generator.
@@ -254,6 +254,7 @@
 		g.P("},")
 	}
 	g.P("},")
+	g.P("Metadata: ", file.VarName(), ",")
 	g.P("}")
 	g.P()
 }