[Inputs using file registry] Improve metrics of file registry

**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

Start by locating the input file-registry implementation, Beat metrics, and Elastic Integration diagnostics paths; review how an input ID identifies each registry and what registry data is already available. Define the supported per-registry statistics from the issue, then verify that metrics and diagnostics expose file counts and oldest/newest timestamps, resolving the histogram and additional-statistics scope with maintainers.

Written by the indexing model from the issue text.