improve registerMetric in orderExecutor

[TakaHiR07]

Main Issue: https://github.com/apache/bookkeeper/issues/4373

Motivation

As is shown in the issue, This is the second pr to improve the registerMetric() in orderExecutor.

Changes

1. remove the duplicate metric
2. modify the metric name format, make it compatible with previous version.

before :

after improvement, the metric is :