There's no simple answer here, but I like to think of developer advocacy/evangelism/relations/etc as an extension of one (or more) of your company's core functions: product, sales, marketing and support. When I was a developer evangelist at Twilio, my team supported the Marketing org, so the team goals aligned with Marketing goals (driving signups).
Often you'll encounter a team of "Developer Advocates". The Google Chrome org has a team of Developer Advocates for example. These folks tend to align more with the Product organization, and their goals/metrics likewise align with the broader goals/metrics of the Product org.
You can use the baseline metrics associated with your functional groups (i.e. top of funnel signups for Marketing) and then layer on metrics that capture the impact that the team is having in isolation from the overall organization. Example: developers who signed-up for your product because an evangelist gave a talk at a conference.
The short answer is that you need to strive to meet developers where they are, rather than trying to draw them to you. This mean: meetups, conferences, online developer communities, etc. Lately, Twitch seems to be a place more and more developers are hanging out.
Events are an easy place to waste money, but you can't do without them. So it's really about being as lean and scrappy as possible, and being creative about how you can add value to an event beyond giving them money.