Add std_dev aggregation for metric visualization

[joejuzl]

Kibana version:
6.1.1

Elasticsearch version:
6.1.1

Describe the bug:

This exact bug #13267 seems to have recurred in 6.1.1

[timroes]

Just for the information why this vanished: We merged gauge and metric implementation in 5.6, but had to revert it again in 6.1, due to several issues, so that got lost by reverting that implementation again.

[joejuzl]

Thanks for in the info @timroes. Just to clarify after the title change - I think the list of missing aggs is: std_dev, percentiles, percentile_ranks, serial_diff, moving_avg, cumulative_sum.

[wylieconlon]

I assume that for a metric visualization we would want the single-value extended_stats.std_deviation, while the default "Standard deviation" aggregation today includes upper and lower bounds. I can imagine two potential fixes:

• Always use the single-value standard deviation, which should enable it in the Metric vis
• Implement a second kind of Std deviation aggregation which is single-value

[stratoula]

This is possible in Lens metric