[self instrumentation] apm-server service missing in aggregated metrics

**APM Server version** (`apm-server version`): 9.3.1

**Description of the problem including expected versus actual behavior**:

expected: apm-server should be on APM UI service summary when self instrumentation is enabled
actual: apm-server is missing. May appear for a short while when UI was using raw traces when aggregated metrics are not ready. But once aggregated metrics are present for other services, apm-server disappears from APM UI.

**Steps to reproduce**:

Please include a *minimal* but *complete* recreation of the problem,
including server configuration, agent(s) used, etc. The easier you make it
for us to reproduce it, the more likely that somebody will take the time to
look at it.

1. Enable self instrumentation
2. Send telemetry to apm-server (e.g. using intakev2 / sendotlp)
3. Observe that apm-server is missing in APM UI

**Provide logs (if relevant)**: