draft: return histograms for time metrics

[Keksoj]

Up until now, time metrics were returned by Sōzu, and rendered in the CLI, as percentiles:

┌────────────────────────┬─────────┬──────┬──────┬──────┬───────┬────────┬─────────┬──────┐
│ Percentiles            │ samples │ p50  │ p90  │ p99  │ p99.9 │ p99.99 │ p99.999 │ p100 │
├────────────────────────┼─────────┼──────┼──────┼──────┼───────┼────────┼─────────┼──────┤
│ accept_queue.wait_time │ 6       │ 0    │ 0    │ 0    │ 0     │ 0      │ 0       │ 0    │
├────────────────────────┼─────────┼──────┼──────┼──────┼───────┼────────┼─────────┼──────┤
│ epoll_time             │ 40      │ 1000 │ 1001 │ 1001 │ 1001  │ 1001   │ 1001    │ 1001 │
├────────────────────────┼─────────┼──────┼──────┼──────┼───────┼────────┼─────────┼──────┤
│ event_loop_time        │ 40      │ 0    │ 0    │ 4    │ 4     │ 4      │ 4       │ 4    │
├────────────────────────┼─────────┼──────┼──────┼──────┼───────┼────────┼─────────┼──────┤
│ response_time          │ 6       │ 0    │ 0    │ 0    │ 0     │ 0      │ 0       │ 0    │
├────────────────────────┼─────────┼──────┼──────┼──────┼───────┼────────┼─────────┼──────┤
│ service_time           │ 6       │ 0    │ 0    │ 0    │ 0     │ 0      │ 0       │ 0    │
└────────────────────────┴─────────┴──────┴──────┴──────┴───────┴────────┴─────────┴──────┘

These percentiles are not convertible for a prometheus layer, however.

Since the time metrics are collected by sozu_lib's LocalDrain as hdrhistograms, why not return them in a histogram form as well? This would allow the Sozu prometheus connector to convert them to prometheus metrics.

This PR creates the protobuf type FilteredHistogram. Now, for each time metric:

1. a Percentiles metric is produced, for instance response_time
2. a FilteredHistogram is produced, named response_time_hist

[Keksoj]

Check issue #751 for inspiration

[Keksoj]

Actually I need to rebase the branch of PR #1004 into this one. #1004 refactors the display of metrics in a much cleaner way, and allows a way better display of JSON. This is crucial for development purposes.