I've never done this before, but I can picture two different approaches, I'm sure there are others.
1) Use sensors to identify activity from Java classes that deal specifically with HTTP 2. For example, the Apache HTTP client just got HTTP2 support. So if you're using that library, add sensors to ApacheClient classes added for HTTP 2 support. To find HTTP libraries that support HTTP 2, check out this page.
2) Use Business Transactions to look at Web Request Session Attributes that indicate HTTP 2 is in use. Open your browser to this AppMon doc page. ...and look for the section titled "Define a Splitting Business Transaction Based on Web Request Session"