What's your process for figuring out what metrics to hold PMM accountable for?

This ties back to business objectives (corporate level KPIs), and how your team / individual role & responsibility is structured against those objectives.

You'll often see that, depending on the company stage and maturity, PMM will skew towards alignment with either Product OR Sales. But it's rarely perfectly positioned in the middle. 

• Let's say your business has an aggressive product growth target... well then you're likely to staff a PMM that'll specialize in launches, or maybe even a lifecycle marketer (the next hottest role after PMM in marketing, I might add). In that case the metric to hold this particular PMM accountable to is: Activation, adoption, or engagement rate within the first 5, 10, 30 days post-launch -- something to that effect.
• On the other hand, if there is a clear sales target, then you could have the PMM aligned to: Revenue, win rate, close rate, ASP, etc.

I recommend to stick with 1 metric wherever possible to not muddy the water. And again, enure alignment with the business objective and take advantage of the fact that most PMM teams get a comprehensive view of the business and can position against critical initiatives (and associated metrics).

Messaging and revenue. But to be clear, here are the specifics:

• Drive pipeline and top-line revenue growth, inclusive of new logo and cross-sell / up-sell (land & expand growth)
• Partner with enablement to ensure quota-carrying teams know what to say to whom and when
• Bring customer feedback into the product and technology team in order to inform product roadmaps.
• Prioritize, open, and adapt to new markets and industries
• Drive home differentiated and unique value