[Inputs using file registry] Improve metrics of file registry

[lucabelluccini]

Describe the enhancement:

It would be great to expose the registry stats of the inputs which use the registry in the Beat metrics and also expose them in the Elastic Integration diagnostics.

Describe a specific use case for the enhancement or feature:

When troubleshooting, it would be interesting to get, for each registry used by each input (use input id to track it?):

• Number of files in the registry
• Older and Newest timestamp of the files in the registry
• Nice to have but IDK if possible: histogram of file permanence time in registry
• Other useful stats for troubleshooting

[elasticmachine]

Pinging @elastic/elastic-agent-data-plane (Team:Elastic-Agent-Data-Plane)

[pierrehilbert]

Would it be a valid solution to add those in https://github.com/rdner/filebeat-registry-analyser?

[cmacknz]

I think we should add these to the beat's /stats output where they are more easily consumed and we have the ability to see them over time.