server-timing -- change to server-metrics and allow arbitrary metrics

(apologies if this is the wrong way to comment on
https://w3c.github.io/server-timing)

It would be nice if the SERVER-TIMING header spec allowed for other metrics
apart from duration. I'd like to include counts of db calls, cache
hits/misses, basically the ability to add arbitrary metrics. I appreciate
duration is useful for drawing timelines, but other metrics would be useful
during development.

SERVER-TIMING database;dur=300;count=10
SERVER-TIMING cache;hit=10;miss=2;duration=100;avg=10;stddev=4.5

Which leads to the possibly over-the-top request to change SERVER-TIMING to
SERVER-METRICS with duration being a pre-defined metric.

Cheers

Steve