[receiver/hostmetrics] Enable re-aggregation feature for memory scraper

Part of #46386.

Enable the re-aggregation feature for the memory scraper in hostmetrics receiver by:

1. Setting reaggregation_enabled: true in metadata.yaml
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