[8.14](backport #4187) [Host metrics] Add field calculation to serverless docs

[mergify]

This PR adds the missing field calculations to the serverless docs

Fixes #3897

Preview link: https://elastic-dot-co-docs-production-8h6qfqwwb-elastic-dev.vercel.app/current/serverless/observability/host-metrics

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This is an automatic backport of pull request #4187 done by Mergify.

[mergify]

Cherry-pick of 2cef44f has failed:

On branch mergify/bp/8.14/pr-4187
Your branch is up to date with 'origin/8.14'.

You are currently cherry-picking commit 2cef44f9.
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  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by us:   docs/en/serverless/infra-monitoring/host-metrics.mdx

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