Add cli flag --version to show version of spanner-cli (#198)

Author: kyu08

README.md

@@ -53,6 +53,7 @@ spanner:
       --directed-read=         Directed read option (replica_location:replica_type). The replicat_type is optional and either READ_ONLY or READ_WRITE
       --skip-tls-verify        Insecurely skip TLS verify
       --proto-descriptor-file= Path of a file that contains a protobuf-serialized google.protobuf.FileDescriptorSet message to use in this invocation.
+      --version                Show version of spanner-cli
 
 Help Options:
   -h, --help                   Show this help message

main.go

@@ -23,6 +23,7 @@ import (
 	"os"
 	"os/user"
 	"path/filepath"
+	"runtime/debug"
 
 	pb "cloud.google.com/go/spanner/apiv1/spannerpb"
 	flags "github.com/jessevdk/go-flags"
@@ -49,6 +50,7 @@ type spannerOptions struct {
 	DirectedRead        string `long:"directed-read" description:"Directed read option (replica_location:replica_type). The replicat_type is optional and either READ_ONLY or READ_WRITE"`
 	SkipTLSVerify       bool   `long:"skip-tls-verify" description:"Insecurely skip TLS verify"`
 	ProtoDescriptorFile string `long:"proto-descriptor-file" description:"Path of a file that contains a protobuf-serialized google.protobuf.FileDescriptorSet message to use in this invocation."`
+	Version             bool   `long:"version" description:"Show version of spanner-cli"`
 }
 
 func main() {
@@ -64,6 +66,11 @@ func main() {
 		exitf("Invalid options\n")
 	}
 
+	if gopts.Spanner.Version {
+		fmt.Fprintf(os.Stdout, "%s\n", versionInfo())
+		os.Exit(0)
+	}
+
 	opts := gopts.Spanner
 	if opts.ProjectId == "" || opts.InstanceId == "" || opts.DatabaseId == "" {
 		exitf("Missing parameters: -p, -i, -d are required\n")
@@ -197,3 +204,11 @@ func readStdin() (string, error) {
 		return "", nil
 	}
 }
+
+func versionInfo() string {
+	info, ok := debug.ReadBuildInfo()
+	if !ok {
+		return "(devel)"
+	}
+	return info.Main.Version
+}