Added cpu_usage_total measurement in CPU input plugin

[d-ulyanov]

CPU input plugin: added cpu_usage_total measurement. This measurement is often used and it would be convenient to have possibility to enable it in config.

[danielnelson]

Won't the usage have poor accuracy because you are adding up percentages instead of saving the raw values and then computing a single percentage?

I think there is another problem with how we loop over the CPUs, but only save one cpuCoreUsage instead of one per cpu plus one for total cpu.

[d-ulyanov]

Accuracy is based on CPU usage % that already reported, so here we have the same accuracy.

I have two root causes for adding this measurement:

1. On graphs where we have several hosts - we don't need to have CPU distributions and per-core usage. It's very convenient to have pre-calculated measurement.
2. We can't build CPU usage quantiles for cluster from histograms based on several CPU distributions :) We need one consolidated measurement. For these histograms we're using The histogram aggregator plugin was added. #2387
   It's cool feature and we're using it for checking cluster states and capacity planning.

To build such graph we need following measurements:

cpu_usage_total{cpu="cpu-total"} 4.485887096501215

cpu_usage_total_bucket{cpu="cpu-total",le="1.0"} 0
cpu_usage_total_bucket{cpu="cpu-total",le="2.0"} 167
...
cpu_usage_total_bucket{cpu="cpu-total",le="90.0"} 32694
cpu_usage_total_bucket{cpu="cpu-total",le="95.0"} 32694

All of them are result of combining #2387 and this PR.

[danielnelson]

@goller Can you review the usage calculation accuracy?

@d-ulyanov Definitely will need to fix the cpuCoreUsage part, we should have stats for all cores and right now it's calculating across cores.

[vlamug]

@danielnelson , could you tell us when you plan to release this request?

[danielnelson]

Can't we more easily add this by just essentially reverting 10b0438?

On the histogram use case, what if you created the histograms using cpu-idle and then invert them in the query since total = 100 - idle?

[danielnelson]

We added cpu_time_active and cpu_usage_active in #2943.