reply_time(latency) field

[neopaf]

Measure time between request and reply and add it as evaluated field, code:

f.reply_time = ProtoField.relative_time("tarantool.reply_time", "Reply time")

local s,nsfrac = math.modf(request_time - response_time)
local delta = NSTime(s, nsfrac*1e9)
subtree:add(f.reply_time, delta):set_generated() end

neopaf#7

[neopaf]

in my task I've fixed that. feel free to take that