[Lens] Thresholds: chart bounds should be automatically extended to include threshold lines

[dej611]

Describe the feature:

Dynamically change the visualization bounds based on the threshold value.

[elasticmachine]

Pinging @elastic/kibana-vis-editors (Team:VisEditors)

[ghudgins]

CC @elastic/datavis for their thoughts on this....I personally think it makes sense

[nickofthyme]

This is good idea but I'd say it should be non-default behavior. Focusing the chart to the extents of the data is more important to me and including the threshold only if applicable.

[markov00]

As already proposed in a meeting, annotations like thresholds can be treated as actual chart data, so we should show the threshold line and increase the boundaries of the chart when required. We can provide the ability to visualize the threshold in the legend and add the ability to hide/show them to restore the, eventually, extended domain

[ghudgins]

++ thanks @markov00 - these 2 issues would help provide more value to thresholds in this case but also generally elastic/elastic-charts#299 & elastic/elastic-charts#555

[flash1293]

We can provide the ability to visualize the threshold in the legend and add the ability to hide/show them

This would be really cool!

[flash1293]

We decided to go forward with including thresholds in the auto bounds for now (custom bounds will be respected). Once the linked elastic-charts features are added, we can introduce them on top in a separate task.