Follow up from #7101. With elastic/beats@84bf434, libbeat introduced a safeguard check before ingesting data to ES. It adds the IsAlias() function to the IndexSelector Interface, enabling the output handler to require an alias instead of an index when ingesting data.

The APM Server currently always returns false for the IsAlias() check for two reasons.

• The check for IsAlias is not straight forward when returning the autoSelector. A fist pass on this can be found in 5111d3e. This implementation does not yet handle whether an alias of index is set in the meta information.
• When ILM is enabled, APM Server still needs to write to indices for sourcemaps and onboarding. Therefore the newly introduced check cannot simply be leveraged by APM at this point.

Investigate how this check can be leveraged for general event ingestion, while still allowing to send sourcemap and onboarding data to indices.