Fix options reading in history purge command.

The added code makes it easier to read CLI options. It's probably a
good idea to refactor that in a function wrapper.

cmd/server/app/history_purge.go

@@ -28,14 +28,19 @@ var historyPurgeCmd = &cobra.Command{
 }
 
 func historyPurgeCommand(cmd *cobra.Command, _ []string) error {
-	batchSize := viper.GetUint("batch-size")
-	dryRun := viper.GetBool("dry-run")
-
+	// TODO: This statement and the following could be refactored
+	// in a function similar to `GRPCClientWrapRunE`
+	if err := viper.BindPFlags(cmd.Flags()); err != nil {
+		return fmt.Errorf("error binding flags: %s", err)
+	}
 	cfg, err := config.ReadConfigFromViper[serverconfig.Config](viper.GetViper())
 	if err != nil {
 		cliErrorf(cmd, "unable to read config: %s", err)
 	}
 
+	batchSize := viper.GetUint("batch-size")
+	dryRun := viper.GetBool("dry-run")
+
 	ctx := serverconfig.LoggerFromConfigFlags(cfg.LoggingConfig).WithContext(context.Background())
 
 	// instantiate `db.Store` so we can run queries