[discover] Add performance metrics

[mattkime]

Add performance metrics:

• Time it takes to load all data (this is proxy for query-time)
• Time it takes until all data is on screen and the page is at rest
• Should also include how the load got triggered (time range change, refresh, page load)
• Add “rendered with data” event similar to logs explorer one
• Enhance request tracking in Discover - Davis might have more details

NOTE: This needs refinement. Discover already has some tracking. We should look at what exists already, see how it fits our current needs and consider changes. This is something that's been important to Dashboards so it might be good to talk to someone who's worked on its performance metrics.

[elasticmachine]

Pinging @elastic/kibana-data-discovery (Team:DataDiscovery)