[Lens] Support multiple thresholds on bar/line/area charts

[sherry-ger]

Describe the feature:
Support multiple thresholds on bar/line/area charts. Thresholds can be lines or rectangles.

Describe a specific use case for the feature:
There are occasion you may need multiple threshold lines, say upper and lower limits.

[elasticmachine]

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

[wylieconlon]

@AlonaNadler This seems to be a request specifically for the default visualize editor, which already supports a single threshold line. It's likely we want a similar feature in Lens, but I consider them separate.

[wylieconlon]

I'm proposing that we limit the scope of this issue to only constant-value thresholds. Dynamic thresholds are possible, but it doesn't seem like a high priority based on my competitive analysis.

I'm expecting that each threshold will be a separate layer, using a layer-type selector that contains these options (my own mockup, not final):

There are two types of thresholds:

• Line
• Bounded or unbounded rectangle

Here are the form elements that I think we will want to have for each type. "Value mode" is not required if the only mode is "Constant value", but it helps to differentiate the bounded/unbounded rectangles.

Using these options users could create colored ranges to indicate their chart health, like this:

[dej611]

Closed via #108342