[Observability metrics reference] Add serverless to the frontmatter

[alaudazzi]

This PR adds the serverless to the frontmatter.
Relates to https://github.com/elastic/docs-projects/issues/432.

[lcawl]

I wonder if this file shouldn't just be removed since it duplicates https://github.com/elastic/docs-content/tree/main/reference/data-analysis/observability/index.md?

Then just update the frontmatter in index.md to make it clear it replaces two pages:

---
mapped_pages:
  - https://www.elastic.co/guide/en/observability/current/metrics-reference.html
  - https://www.elastic.co/guide/en/serverless/current/observability-metrics-reference.html
 applies_to:
  stack: all
  serverless: all
---

Actually that applies_to is probably unnecessary, since it's asserting what should generally be the default (i.e. the page applies to everything).