[receiver/hostmetrics] `systemscraper` has some references to its old name

[braydonk]

Component(s)

receiver/hostmetrics

Describe the issue you're reporting

When I renamed the systemscraper, I neglected a few spots where the old uptimescraper name is still referenced in comments and some function names.

[github-actions]

Pinging code owners:

• receiver/hostmetrics: @dmitryax @braydonk

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