[Lens] Linear regression

[flash1293]

It should be possible to easily show a linear regression line in Lens for any given series.

Unclear parts:

• Where should this option be located? Maybe as an option in the dimension configuration flyout?
• Where should this calculation be performed? Is it general enough for an integration in elastic-charts?

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)

[ghudgins]

+1 from a user who is doing this in Timelion but wants the setup to be easier:

.es(index=aaa,
    timefield='start',
    metric='percentiles:values.out_bits.rate_hist:99')
    .label('In Bits(99%tile 30 sec)'),
.es(index=aaaa,
    timefield='start',
    metric='percentiles:values.out_bits.rate_hist:99')
    .trend(mode=linear)
    .label('Regression')

[timductive]

Closing in favor of #159780