[APM] Enable ability to provide custom renderers for span labels shown in span details flyout

[spong]

In #171153 we instrumented the Security Solution Assistant with APM so we could trace and debug our calls to LLM's.

This is an enhancement request for enabling the ability to provide custom renderers for a span's label when viewed in the span details flyout. For instance, the labels.inputs field below is a list of all the knowledge base documents returns from a similarity search during the retrieval phase of one of the chains. In this instance it would be nice to have a custom view here that would only show the knowledge base document's title, but then allow the user to expand to see the full contents.

[elasticmachine]

Pinging @elastic/apm-ui (Team:APM)

[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.