[Infra UI] no longer update metricsAlias field in infrastructure-ui-source

[neptunian]

In #164094, we created a new saved object called metrics-data-source with the field metricIndices. We use this as our source of truth when querying for the user's metrics data source, which they change in the Infra UI Settings page. We continue to fall back on where we previously stored it, infrastructure-ui-source in field metricsAlias. See (#161876 (comment)) for illustration.

This issue calls for a few tweaks to the behaviour of this flow:

• Stop setting the metricsAlias field and value as part of the default configuration in the infrastructure-ui-source saved object. It should still be part of the default configuration in the code but not the SO. The SO is first created when a user saves their settings for the first time. We'll keep this field as a type on the infrastructure-ui-source for backwards compatibility but otherwise we don't need to set it.
• Don't save the field to the infrastructure-ui-source saved object when user saves the settings since its already being saved in metrics-data-source. For users in the fallback scenario, once its been "migrated" or saved to the metrics-data-source plugin when they save their settings again, we can unset the field in infrastructure-ui-source.
• import the default METRICS_INDEX_PATTERN into infra instead of having it in infra because the source of truth should live in the metrics-data-source plugin. We still need this as part of the default source configuration used before a user creates the saved object

I believe these changes will make the code more understandable and less redundant as we are currently storing and updating the source in two places.

[elasticmachine]

Pinging @elastic/observability-asset-management (Team:Observed Asset Management)

[elasticmachine]

Pinging @elastic/infra-monitoring-ui (Team:Infra Monitoring UI)

[elasticmachine]

Pinging @elastic/obs-ux-infra_services-team (Team:obs-ux-infra_services)

[botelastic]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.