service.name missing from intake metadata

## Describe the bug

As part of the ops KPI review we've been investigating the following errors:

```
validation error: metadata: service: 'name' required
```

It appears the ruby agent is sending metadata without the `service.name` field.

## Steps to reproduce

I don't have steps to reproduce, this is part of the ops KPI review and was observed in the elastic cloud logs.

## Expected behavior

Data is reported correctly and ingestion succeed

## Environment

The error has been observed in the following agent versions:
- elastic-apm-ruby/3.10.1 http.rb/4.4.1 ruby/2.6.6
- elastic-apm-ruby/4.5.0

This happens on multiple apm-server versions.

## Additional context
none