You can't change the service name with a plugin, but you can define custom service names in the settings. Here is the Documentation. The service names are highly configurable that way.
What would be the use case to do it with a plugin, and not in the settings?
Again, what would would be a possible change you would like to make?
If you are missing Metrics you can add additional ones per Plugin or API, depending on the use case.
Yes if the default settings/capabilities are not enough for you, you can always extend the functionallity with the SDK. But the first step should always be to try it with the default OneAgent capabilities.