As discussed in one of my last blog posts, we really like New Relic for performance metrics and use it a lot. Unfortunately there isn't an extension for HHVM (yet) and HHVM is becoming an important part in our setup. But - a big great coincidence - New Relic released an Agent SDK and with that, an example extension for HHVM and WordPress. That was a great start for me to get behind the whole thing.
He talks about writing a HHVM extension and includes an example of the implementation. Christian also talks about the challenges around profiling data and finding out where the requests "spend their time" in the execution. There's two solutions he suggests, but they each have their tradeoffs (a recompiled/patched version or a performance hit). He provides the extension they've built in this github repository.