You can use zipkin to gather stats about the functions server.
Running a zipkin node is easy to get started, they have a docker container:
With zipkin running you can point functions to it using an env var:
ZIPKIN_URL=http://zipkin:9411/api/v1/spans
TODO hook up zipkin to poop out to logs/statsd/something else too
The Logspout Statsd Adapter adapter can parse the log metrics and forward them to any statsd server.