Document count_* metrics

[greatvovan]

In my Grapfana+Graphite+statsd installation I see in the UI of time series of a timer:

This may be confusing for newcomers.

1. What is count_ps? Only in some discussions and issues I found that it is "count-per-second". In the documentation there is no a word about it.

2. What is the meaning and use of count_90 and count_99? I also suggest to add it to documentation.

[BlueHatbRit]

Hi @greatvovan, you're right we're lacking some documentation around these, they're being worked on but likewise if you'd like to contribute then that would be awesome!

count_ps is indeed count-per-second which is calculated in addition to your flush interval. Count_90 and Count_99 refer to the 90th and 99th percentile, which we also give with a few of the other metric types as well. It's a great way to understand what's happening often vs rarely in a system.

If you're not familiar with 90th, 99th percentiles in metrics then there's a good explanation here - https://stackoverflow.com/questions/38781499/what-does-90th-95th-99th-pct-matrices-means-in-dashboard-report-of-jmeter.

Hopefully I'll get some time to do a documentation pass over the next few months, things are quite busy for me at the moment though and I seem to be the primary maintainer these days.

[greatvovan]

I absolutely can do that, but first of all I need to understand it myself.

@BlueHatbRit I know what are percentiles, but I have no idea how to apply this concept to counters. Say I have 10 values:

4 3 5 20 3 3 4 5 30 4

Percentiles consider the values and 90 percentile will cut the highest two (20 and 30). But as for counters, it will work regardless of values, and the result will be always 8. It looks like count_90 is always equal to count * 0.9 and count_99 == count * 0.99.

If the above is correct, I cannot even imagine what can be the use of such metrics.