POC fast historgam

[yngvar-antonsson]

POC for #461

I moved the code from gist to histogram.lua and kept the old histogram in histogram_old.lua. I keep getting the same result for both histograms on my Intel MacBook. Maybe I did smth wrong?

ho = require('metrics.collectors.histogram_old')
api_timings_hist = ho:new('old')
hf = require('metrics.collectors.histogram')
api_timings_fast = hf:new('new')
clock = require('clock')

tarantool> clock.bench(function() local lbls = {method='api.users.get',ok='true'} local prev = clock.time() for _ = 1, 1e6 do api_timings_hist:observe(clock.time()-prev, lbls) prev = clock.time() end end)
---
- - 34.845349
...

tarantool> clock.bench(function() local lbls = {method='api.users.get',ok='true'} local prev = clock.time() for _ = 1, 1e6 do api_timings_fast:observe(clock.time()-prev, lbls) prev = clock.time() end end)
---
- - 34.938217
...

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