ctlv3: handle pkg/flags warnings

Signed-off-by: Gyuho Lee <gyuhox@gmail.com>

etcdctl/ctlv3/command/global.go

@@ -101,8 +101,19 @@ type clientConfig struct {
 	acfg             *authCfg
 }
 
+type discardValue struct{}
+
+func (*discardValue) String() string   { return "" }
+func (*discardValue) Set(string) error { return nil }
+func (*discardValue) Type() string     { return "" }
+
 func clientConfigFromCmd(cmd *cobra.Command) *clientConfig {
 	fs := cmd.InheritedFlags()
+
+	// silence "pkg/flags: unrecognized environment variable ETCDCTL_WATCH_KEY=foo" warnings
+	// silence "pkg/flags: unrecognized environment variable ETCDCTL_WATCH_RANGE_END=bar" warnings
+	fs.AddFlag(&pflag.Flag{Name: "watch-key", Value: &discardValue{}})
+	fs.AddFlag(&pflag.Flag{Name: "watch-range-end", Value: &discardValue{}})
 	flags.SetPflagsFromEnv("ETCDCTL", fs)
 
 	debug, err := cmd.Flags().GetBool("debug")