[receiver/hostmetricsreceiver] Enable re-aggregation feature

Part of #45396.

Enable the re-aggregation feature for the hostmetrics receiver scrapers by:

1. Setting reaggregation_enabled: true in metadata.yaml for each scraper
2. Ensuring all attributes are properly defined with appropriate levels:
   • required: cannot be reaggregated
   • recommended: enabled by default but can be disabled by users
   • opt_in: disabled by default but can be enabled by users
3. Validate the feature after the feature gate is enabled: all existing tests pass, manual check

Scrapers to update:

• [x] cpu #46614
• [x] disk #46615
• [x] filesystem #46616
• [x] load #46617
• [ ] memory #46618
• [ ] network #46619
• [ ] nfs #46620
• [ ] paging #46621
• [x] processes #46622
• [ ] process #46623
• [ ] system #46624