[Reporting/Maps] Background tiles in a map panel might not load in a screenshot report

[tsullivan]

Kibana version: 7.9+

Original install method (e.g. download page, yum, from source, etc.): ECE

Describe the bug:
A Map panel waits for ES data to load and rendering of the data to complete, but it isn't aware of the requests for the background tiles that are also necessary for rendering.

When all the data is loaded and rendered in the panel, a dashboard panel gives itself an attribute in the HTML code that Reporting can detect. The Map panel updates the HTML code for Reporting, without knowing about the loading state of the background tiles. Therefore, a timing issue exists where Reporting may take a screenshot of the page before it should.

Steps to reproduce:

1. Use 2-4GB of RAM for Kibana instance in ECE
2. The Kibana instance should not have outside access to the Internet
3. Use proxy settings to have Reporting's Chromium browser access the map tile server:

   xpack.reporting.capture.browser.chromium.proxy.enabled: true
   xpack.reporting.capture.browser.chromium.proxy.bypass: ["*.elastic.mycompany.com","*mycompany.com","0.0.0.0"]
   xpack.reporting.capture.browser.chromium.proxy.server: https://proxy.mycompany.com:8088
   
   # in a constrained environment, there needs to be extra wait time for screenshotting events
   xpack.reporting.capture.timeouts.renderComplete: 240000
   xpack.reporting.capture.timeouts.waitForElements: 240000
   
   # extra wait time for the report job
   xpack.reporting.queue.timeout: 240000
   

4. The map tiles will be requested through the proxy, which adds a bit of latency.

Expected behavior:
The map should have image tiles rendered behind the data.

Instead, the data is present but the tiles are blank.

Screenshots (if relevant):

Errors in browser console (if relevant):
No errors

Provide logs and/or server output (if relevant):

Any additional context:
Side note: in one case, PDF reports consistently load with all the background tiles, whereas PNG reports sometimes look OK, but sometimes have missing background tiles. It seems there is a detail about the implementation that could be making the difference, and that merits further investigation.

[elasticmachine]

Pinging @elastic/kibana-app-services (Team:AppServicesUx)

[elasticmachine]

Pinging @elastic/kibana-gis (Team:Geo)

[nreese]

@thomasneirynck added vector tile loading status. This status can be used to let reporting know when base map vector tiles have been loaded.