[Discover] Report kibana:plugin_render_time metric event.

[kertal]

The metric kibana:plugin_render_time measures the time from the start of navigation to the point at which the most meaningful component appears on the screen.

https://docs.elastic.dev/kibana-dev-docs/tutorial/performance/adding_custom_performance_metrics#how-it-works

What's the most meaningful component on the screen? One could argue, it's when data is rendered in the Data Table

kibana/src/plugins/discover/public/components/discover_grid/discover_grid.tsx

Lines 20 to 30 in 339379e

	*/
	export const DiscoverGrid: React.FC<UnifiedDataTableProps> = (props) => {
	return (
	<UnifiedDataTable
	showColumnTokens
	enableComparisonMode
	renderCustomToolbar={renderCustomToolbar}
	{...props}
	/>
	);
	};

However, this would not cover the following:

• No data state
• Error data state
• Legay table
• No automatic start of data fetching
• View surrounding documents view
• View single documents state

So it would need some research if the most meaningful component could be the data table, or if there are alternatives.

[elasticmachine]

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