It took me a while to figure out what is the difference in these two metrics and one day I was explained in a very nice way, which was embedded in my mind.
Success Metric - A metric which the CEO or CMO would care about. Example: Total new clients or revenue
Diagnostic Metrics - A metric which is used to explain or diagnose the success metric if it over achieved or under achieved. Example: Click-through rate, Interaction rate, Landing page visits
I feel it is very important to put them in the measurement plan and also realize if it is feasible to calculate them. The reason I talk about feasibility is that many time due to technological limitations, it is not possible to get all the data required to calculate the metrics.
For example our friends at Google do not accept third party vendors (example: DART) to serve display ads (banner) and thus it is hard to get the number of impressions and clicks or any other post-impression, post-click activity for the banners served through the Google ad network. Yes, I do know that Google just bought DART - and trust me until this day, they do not accept third party tags.
I have been in various situation where earlier it seems feasible to get the data but when the campaign really launches, it becomes hard or impossible to get the data. OR sometimes the correct tags are not put in place which enable to get the data in the correct form.