[Lens] Investigate how Lens could allow variables to be passed in from the dashboard

[wylieconlon]

The idea of variables that control parts of a dashboard has been proposed many times in different Kibana issues, but we do not have an implementation plan that we can use to integrate it into Lens. We can't implement variables in Lens without considering the end-to-end user experience and technical questions that come up.

Previously written use cases:

1. Switching the index pattern- there is some discussion, but I didn't see an answer to the question of "how does this integrate?". Not all index patterns have compatible fields.
2. Users can define variables in the dashboard- what kind of variables?
3. Assign variables in the TSVB, Timelion editors for use in dashboards
   • Lots of example use cases here
4. Parameterized panels in dashboards, repeat for each field value
5. Conditionals in dashboards

Lens-specific use cases:

5. Replace a specific field with another field
6. Replace one metric with another
   • Example: Switch from Last Value to aggregate
7. Threshold line with a value from the dashboard

I'm making a few assumptions, which I want to document here:

1. Variables are opt-in, probably based on typing variable names
2. Variables need to be validated by the app, Lens has its own rules
3. There is probably a hierarchy of variables
4. Variables might represent complex types, like an index pattern and all of its fields
5. Not all variables have a default value, such as a fully dynamic visualization

Open questions:

• What happens if a variable is missing?
• What happens if the user types an invalid value for the variable, like an index pattern that doesn't have the right fields?
• ...

cc @elastic/kibana-presentation

[elasticmachine]

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

[markov00]

In order to provide better transparency of priorities, issues that will not be prioritized within the next 24 months are being closed.

Tracking request in Lens general improvements ice box #184459