OpenTelemetry spans received from ADOT appear as having different `service.name` and coloured differently

**APM Server version** (`apm-server version`): 7.13.1

**Description of the problem including expected versus actual behavior**: I am collecting OpenTelemetry traces from AWS Distro for OpenTelemetry Lambda using the APM server OTel intake and the recorded spans are coloured, potentially because they don't have the OTel equivalent of the `service.name` field.

**Steps to reproduce**:
Attached:

1. instructions on how to set up Java or Python Lambdas to send the APM data into Elastic Cloud [Environment setup.pdf](https://github.com/elastic/apm-server/files/6637285/Environment.setup.pdf)
2. JSON documents for the transaction and 2 underlying spans [3docs.json.txt](https://github.com/elastic/apm-server/files/6637283/3docs.json.txt).
3. Screenshot of the APM UI with 1 transaction and 2 spans

4. CloudWatch debug events for a *similar* call
[log-events-viewer-result.csv](https://github.com/elastic/apm-server/files/6637381/log-events-viewer-result.csv)