Add grpc check example. (#306)

* Add grpc check example.

Signed-off-by: Bo-Yi Wu <appleboy.tw@gmail.com>

* add missing package.

Signed-off-by: Bo-Yi Wu <appleboy.tw@gmail.com>

vendor/golang.org/x/net/context/context.go

@@ -5,6 +5,8 @@
 // Package context defines the Context type, which carries deadlines,
 // cancelation signals, and other request-scoped values across API boundaries
 // and between processes.
+// As of Go 1.7 this package is available in the standard library under the
+// name context.  https://golang.org/pkg/context.
 //
 // Incoming requests to a server should create a Context, and outgoing calls to
 // servers should accept a Context. The chain of function calls between must