Screenshot Service

[tsullivan]

##Summary
At the lowest level the Screenshot Service will provide the ability for any Kibana plugin to take a screenshot of any Kibana page and return the output as a PNG or PDF.

At a higher level the Expression Screenshot Plugin will provide the ability to take a screenshot of a Kibana expression (be it a Dashboard visualization or Canvas element) and return it as PNG or PDF.

##Problem Statement
Currently, there is no single reliable way to export a Dashboard panel or a Canvas element as a PNG image. Currently, there also is no way to take a screenshot of an arbitrary Kibana UI.

We have the Reporting plugin, which internally is able to take screenshots of Kibana UI to generate reports. We would like to expose the screenshot taking ability of the Reporting plugin (the Screenshot Service) as a standalone plugin (the Screenshot Plugin), which would enable other plugins to take screenshots of Kibana UI for various use cases independent of the Reporting Plugin.

There are a number of use cases where we need to be able to create a screenshot of some Kibana UI:

1. Kibana Reporting creates PNG and PDF reports by capturing screenshots of Kibana.
2. Share-to project needs to be able to create PNG and PDF screenshots of Dashboard panels. (Maybe could be extended to Canvas elements, too.)
3. Alerting needs to be able to send visuals in e-mails and Slack messages. The visuals could be a single visualization, or few visualizations (dashboard), or some other visual relevant for the alert.
4. We want to enable solution teams to capture screenshots of Kibana UI.
5. When embedding visualizations in comments, we may want to store visualizations as images, which capture the state of the visualization when it was created.

To solve for all the use cases above, we will create two plugins:

• The Screenshot Plugin — implements the Screenshot Service, can install and use Chromium, knows how to wait until the Kibana page finishes loading, is able to clip specific regions of a page, and is able to return the screenshot as PNG or PDF.
• The Expression Screenshot Plugin — is able to return a PNG or PDF of a Kibana expression (achieves it by using the Screenshot Service).

cc @vadimkibana

[elasticmachine]

Pinging @elastic/kibana-reporting-services (Team:Reporting Services)