[receiver/hostmetrics] Add an uptime scraper

[atoulme]

Component(s)

receiver/hostmetrics

Is your feature request related to a problem? Please describe.

The uptime of the system can be reported by the hostmetrics receiver.

Describe the solution you'd like

We would like to define an additional scraper. It would report periodically the uptime of the machine as reported by gopsutil.

This would be reported as a single metric.

Describe alternatives you've considered

No response

Additional context

No response

[github-actions]

Pinging code owners:

• receiver/hostmetrics: @dmitryax @braydonk

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[ChrsMark]

SemConv PR for this: open-telemetry/semantic-conventions#1507

[braydonk]

I'd propose we call the scraper system. We haven't been calling it a rule before but the way the system semantic conventions have worked out mapped to our hostmetrics scrapers is that the namespace of the metric ends up being the name of the scraper. i.e. system.memory.* will be produced by memoryscraper and so on. So system.uptime being produced by systemscraper would follow the rule and future-proof us against potential future root system metrics (I can't think of any potential examples that might come up but this way we wouldn't be stuck if it happens).
@dmitryax what are your thoughts here?

[kevinnoel-be]

Duplicate of #31627 ?