[APM] Annotations: Service version label could be better

I'm not really sure what to call this issue. I've been playing around with the annotations API, and I'm wondering if we should reconsider the "Service version" graph label and "Version" key in the popup:

When annotations are derived from a change in service.version, this makes sense, but with custom labels on annotations, it might make more sense to remove the "Version" key, and change the "Service version" label to something else.