[Telemetry] Unify Monitoring and Local collectors payload structure

[afharo]

As part of the #57332 I've noticed Monitoring and Local (OSS and X-Pack) collectors differ in the payload structure in some cases:

• One of them is about the license (but already fixed in that same PR).
• The other one I noticed is about the stack_stats.kibana.plugins: Monitoring seems to be merging xpack and non-xpack stats into a flat object
  

  kibana/x-pack/legacy/plugins/monitoring/server/telemetry_collection/get_kibana_stats.ts

  Lines 114 to 117 in 8686fc9

  	// Stats filtered by telemetry collectors need to be flattened since they're pulled in a generic way.
  	// A plugin might not provide flat stats if it implements formatForBulkUpload in its collector.
  	// e.g: we want `xpack.reporting` to just be `reporting`
  	const plugins = { ...pluginsTop, ...xpack };

  
  while X-Pack is reporting them under its own xpack property
  

  kibana/x-pack/legacy/plugins/xpack_main/server/telemetry_collection/get_stats_with_xpack.ts

  Line 19 in 8686fc9

  localStats.stack_stats.xpack = xpack;

Creating this issue to track a further analysis and normalisation of the data.

[elasticmachine]

Pinging @elastic/pulse (Team:Pulse)

[TinaHeiligers]

We need to see if this has any influence on the solutions usages pipelines

[afharo]

I've just retrieved the payloads from both collectors and for the same instance and the structures correctly match. The differences are related to data accessibility (monitoring has access to other products and the local collector knows about nodes' usage and data telemetry).

We can close this issue.

[elasticmachine]

Pinging @elastic/kibana-core (Team:Core)