[receiver/hostmetrics] Rename hostmetrics to host_metrics to match naming convention

[osullivandonal]

Component(s)

receiver/hostmetrics

Describe the issue you're reporting

With the convention for component names established to lower_snake_case in open-telemetry/opentelemetry-collector#14208 and a practical method to migrate existing components implemented in open-telemetry/opentelemetry-collector#14349, this issue proposes renaming the hostmetrics receiver to follow the naming convention.

Proposed change: hostmetrics → host_metrics

This is a sub-issue of #45339

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[github-actions]

Pinging code owners:

• receiver/hostmetrics: @dmitryax @braydonk

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