[APM] Implementation of runtime UI index configuration

[katrin-freihofner]

As described in #39741 this new APM settings page should allow users to configure UI indices. These settings override the kibana.yml settings.

The Settings button in the APM home screen should lead to a new settings page. This page is responsible for configuring the following UI indices:

apm_oss.sourcemapIndices
apm_oss.errorIndices
apm_oss.onboardingIndices
apm_oss.spanIndices
apm_oss.transactionIndices
apm_oss.metricsIndices
apm_oss.apmAgentConfigurationIndex

Prototype:
https://www.figma.com/proto/yeugPdG4YlLNJK6iGPdExu/39741-APM-config?node-id=19%3A1045&viewport=699%2C811%2C0.25&scaling=min-zoom

Additional notes:

• The Apply changes button is disabled until the inputs are actually changed.
• The hints underneath the inputs should state the value from the config file (kibana.yml)
• A toast message should indicate the successfully applied changes (if anything goes wrong we can also use problem toast) https://elastic.github.io/eui/#/guidelines/toasts
• please consider form validation (https://elastic.github.io/eui/#/forms/form-validation) for input fields

The Agent configuration that is currently visible when clicking Settings should move to a separate page which is reachable via the menu on the left.

Text:
The APM UI uses index patterns to query your APM indices. If you've customized the index names that APM Server writes events to, you may need to update these patterns for the APM UI to work. Settings here take precedence over those set in kibana.yml.

[elasticmachine]

Pinging @elastic/apm-ui

[sorenlouv]

@katrin-freihofner When clicking "Settings" which should be the landingpage? UI Indices page or Agent configuration page?

[katrin-freihofner]

My idea was the UI indices - that's also shown in the prototype.

[sorenlouv]

Ah, my bad 👍