Add method to output server-tracing formatted data

[coreylight]

It may be useful for consumers to be able to call a method that formats the trace data that could be easily populated into a header, following the server-timing spec.
https://w3c.github.io/server-timing/

Might be very similar to this:
https://github.com/remy/server-timings/blob/master/index.js#L33