Feature request: introduce time to first byte metrics

[jaronsummers]

It would be really handy to have TTFB data. Because of the nature of bazel caching, it's expected that some requests will take multiple powers of magnitude more than others, so it's somewhat challenging to draw useful conclusions about the cache's performance from the timing/latency data in traces. Having TTFB would give a more reliable view into how long the cache is taking to begin doing its job.

• this was discussed a bit during office hours, and @EdSchouten mentioned https://github.com/buildbarn/bb-storage/tree/master/pkg/blobstore/buffer as a possible place to insert this telemetry.