I have a little experience directly and indirectly managing these kinds of teams. :)

Hard metrics for Developer Relations/Evangelism are super hard to get right. You can easily motivate the wrong behaviors OR suppress the natural enthusiasm that make "great" advocates great.

In my view, there are two things you can measure: Output/Effort and Results/Impact.

Output/Effort is something a Developer Advocate team has complete control over. They can create a plan, execute against it and then be measured by their ability to meet/exceed that plan. The impact of that effort is then overlaid primarily to inform which activities should be repeated in the future, and which should be avoided due to insufficient impact.

Results/Impact is ultimately what the Developer Advocate needs to achieve, but it is also hard for a developer advocate to fully control. A hardworking, talented advocate could do all the right things, but that doesn't guarantee standing-room only audiences and 150,000 followers.

Worse, if you overly judge an Advocate's performance on the later, it can drive odd behaviors in pursuit of  "hitting their numbers." On paper it will look like "big impact," but the metrics won't truly reflect the kind of genuine impact you want Advocates to have.  

Add to that, no two Advocates are the same. Some are GREAT speakers. Some are GREAT writers. Some can do KILLER demos/open source projects. A FEW can do it all. That demands that metrics around DevRel be tailored to the team's strengths if they are going to be effective. 

After almost a decade in this space, the system I feel best about is one that judges 70% to 80% of a Developer Advocate's success on their Output/Effort (versus plan) and 20% to 30% on the impact/public success of those actions.

Butts in seats, Number of conversations at a booth, Number of influencers at a dinner, Twitter followers, Retweets, High-fives...all fine, but not great measures of Developer Advocate success.