[receiver/hostmetrics] restrict system.processes.created to Linux and OpenBSD

[ysmnababan]

Description

Update the documentation for system.processes.created to clarify that the metric is only supported on Linux and OpenBSD.

The runtime implementation already restricts this metric to those operating systems. Since per-metric OS restrictions are not supported in the metadata schema, the metric description was updated to reflect the actual behavior.

Link to tracking issue

Related to #45721

Testing

• Ran make test inside the receiver/hostmetricsreceiver folder
• Ran go generate ./... to regenerate documentation
• Verified the project builds successfully

Documentation

Updated the metric description in metadata.yaml and regenerated the corresponding documentation.

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: ysmnababan / name: ysmnababan (34c868a, 83cd6f9, a04b519)

[github-actions]

Welcome, contributor! Thank you for your contribution to opentelemetry-collector-contrib.

Important reminders:

• Please review our Contributing Guidelines.
• Don't forget to sign the Contributor License Agreement (CLA) if you haven't already.

A maintainer will review your pull request soon. Thank you for helping make OpenTelemetry better!

[rogercoll]

LGTM (see OS guard https://github.com/rogercoll/opentelemetry-collector-contrib/blob/main/receiver/hostmetricsreceiver/internal/scraper/processesscraper/processes_scraper_unix.go#L19)