[SLO] Add preview chart to custom metric indicator

[simianhacker]

Summary

This PR adds a preview chart to the custom metric indicator.

[apmmachine]

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Just comment with:

• /oblt-deploy : Deploy a Kibana instance using the Observability test environments.
• run elasticsearch-ci/docs : Re-trigger the docs validation. (use unformatted text in the comment!)

[elasticmachine]

Pinging @elastic/actionable-observability (Team: Actionable Observability)

[kibana-ci]

💚 Build Succeeded

• Buildkite Build
• Commit: c37a67c

Metrics [docs]

Async chunks

Total size of all lazy-loaded chunks that will be downloaded as the user navigates the app

id	before	after	diff
observability	1.0MB	1.0MB	+27.0B

Unknown metric groups

ESLint disabled line counts

id	before	after	diff
enterpriseSearch	14	16	+2
securitySolution	409	413	+4
total			+6

Total ESLint disabled count

id	before	after	diff
enterpriseSearch	15	17	+2
securitySolution	488	492	+4
total			+6

History

• 💚 Build #141575 succeeded 2183cf1
• 💔 Build #141424 failed 7eba465

To update your PR or re-run it, just comment with:
@elasticmachine merge upstream

[kdelemme]

Nice abstraction 👍🏻