Re: [w3ctag/design-reviews] Review request for Server-Timing (#188)

Would also like to see an explainer; we [recently put together a dummy outline that could help in drafting yours](https://docs.google.com/document/d/1cJs7GkdQolqOHns9k6v1UjCUb_LqTFVjZM-kc3TbNGI/edit?usp=sharing)

Would also be good to see discussion of security and privacy considerations in the draft spec.

In terms of the design, a few questions:

 - The encoding for these values seems very one-off; does it share a common grammar with other systems (e.g., CSP)? Could it possibly use [JSON](http://httpwg.org/http-extensions/jfv.html)?
 - The exact parsing semantics of the `metric-value` (and reflection back to JS) seem ambiguous to me. No formalism (or JS conversion) are offered in the text, only the note that they must be representable as a double, but that leaves a lot of room for interpretation. I'd like to see specific numeric conversion steps outlined.
 - The example code in the spec uses a confusing mix of old and new syntax. A fully modern version might read:

```js
// Example 2
// serverTiming entries can live on 'navigation' and 'resource' entries
['navigation', 'resource'].forEach((entryType) => {
  let entries = performance.getEntriesByType(entryType);
  for(let { name: url, serverTiming } of entries) {
      // iterate over the serverTiming array; only the slow ones
      for(let { name, duration, description } of serverTiming.filter(t => { t.duration > 200 }) {
        console.info('Slow server-timing entry =',
            JSON.stringify({url, entryType, name, duration, description}, null, 2))
      }    
  }
```


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