Enable set the command arguments for plugin

[izturn]

Summary

Add the option to set the command argument(s) for metric/traffic router.

Use Cases

when I implement the traffic router plugin for contour, I found it can't set the log level for the plugin, it leads to too much or fewer logs, so add the option to enable customize it

When would you use this?

---

Message from the maintainers:

Impacted by this bug? Give it a 👍. We prioritize the issues with the most 👍.