Make basicstats faster

[markusr]

Steps to reproduce:

1. Config basicstats to only do

stats = ["count", "min" , "max" , "mean"]

Expected behavior:

"mean" is calculated at the Push by dividing

v.sum / v.count

Actual behavior:

All values also variance are calculated on each Add
https://github.com/influxdata/telegraf/blob/master/plugins/aggregators/basicstats/basicstats.go#L107

Additional info:

I tested this on a raspberry pi 3.

After removing the "online" calculation part the cpu usage dropped from 72% to 35%. (3000 metric per second input and 5s aggregation time).

Another idea would be to add sum and mean to the minmax aggregator or a complete new aggregator.

[danielnelson]

@markusr I think we need the online mean for the online variance, but we should get a big performance gain by switching from Fields() to FieldList(). I made this change in #4741, can you compare the performance with this code?

[markusr]

@danielnelson
You are right. #4741 gives the same speed up. The CPU usage I measured was on the same level (35.41%) as my test before.

It can be merged.