Update host-metrics-receiver.rst

[mabac16]

the latest release included the system scraper in the host metrics receiver which outputs the metric system.uptime (maybe more). please consider changing the documentation on both default configuration files and the receiver documentation.

Requirements

• Content follows Splunk guidelines for style and formatting.
• You are contributing original content.

Describe the change

Enter a description of the changes, why they're good for the Observability Cloud documentation, and so on.

If this contribution is time sensitive, tell us when you'd like this PR to be merged.

[sheilattran8]

Thanks for the update, @mabac16!