etcdctl: support query count only of specified prefix

etcdctl/ctlv3/command/get_command.go

@@ -31,6 +31,7 @@ var (
 	getFromKey     bool
 	getRev         int64
 	getKeysOnly    bool
+	getCountOnly   bool
 	printValueOnly bool
 )
 
@@ -50,6 +51,7 @@ func NewGetCommand() *cobra.Command {
 	cmd.Flags().BoolVar(&getFromKey, "from-key", false, "Get keys that are greater than or equal to the given key using byte compare")
 	cmd.Flags().Int64Var(&getRev, "rev", 0, "Specify the kv revision")
 	cmd.Flags().BoolVar(&getKeysOnly, "keys-only", false, "Get only the keys")
+	cmd.Flags().BoolVar(&getCountOnly, "count-only", false, "Get only the count")
 	cmd.Flags().BoolVar(&printValueOnly, "print-value-only", false, `Only write values when using the "simple" output format`)
 	return cmd
 }
@@ -64,6 +66,12 @@ func getCommandFunc(cmd *cobra.Command, args []string) {
 		ExitWithError(ExitError, err)
 	}
 
+	if getCountOnly {
+		if _, fields := display.(*fieldsPrinter); !fields {
+			ExitWithError(ExitBadArgs, fmt.Errorf("--count-only is only for `--write-out=fields`"))
+		}
+	}
+
 	if printValueOnly {
 		dp, simple := (display).(*simplePrinter)
 		if !simple {
@@ -83,6 +91,10 @@ func getGetOp(args []string) (string, []clientv3.OpOption) {
 		ExitWithError(ExitBadArgs, fmt.Errorf("`--prefix` and `--from-key` cannot be set at the same time, choose one"))
 	}
 
+	if getKeysOnly && getCountOnly {
+		ExitWithError(ExitBadArgs, fmt.Errorf("`--keys-only` and `--count-only` cannot be set at the same time, choose one"))
+	}
+
 	opts := []clientv3.OpOption{}
 	switch getConsistency {
 	case "s":
@@ -159,5 +171,9 @@ func getGetOp(args []string) (string, []clientv3.OpOption) {
 		opts = append(opts, clientv3.WithKeysOnly())
 	}
 
+	if getCountOnly {
+		opts = append(opts, clientv3.WithCountOnly())
+	}
+
 	return key, opts
 }