Remove unused API and resolve naming ambiguities

[raphw]

This PR makes a clear distinction between API and implementation, and cleans up a bit. For this:

• The internal Tracer API is removed.
• Implementation versions of API types are suffixed Impl.
• The ServiceAwareTracer API is consolidated with the Tracer API.
• Minor clean up of dependency imports.

[github-actions]

👋 @raphw Thanks a lot for your contribution!

It may take some time before we review a PR, so even if you don’t see activity for some time, it does not mean that we have forgotten about it.

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[botelastic]

Hi! We just realized that we haven't looked into this issue in a while. We're sorry! We're labeling this issue as stalled to make it hit our filters and make sure we get back to it in as soon as possible. In the meantime, it'd be extremely helpful if you could take a look at it as well and confirm its relevance. A simple comment with a nice emoji will be enough :+1. Thank you for your contribution!