apm server does not send the unit field with metrics.

**APM Server version** (`8.9`):

**Description of the problem including expected versus actual behavior**:
apm server does not send the unit field with metrics.
```
"metricset": {
"name": "app",
"samples": [
{
"name": "system.memory.usage",
"type": "counter",
"value": 5137465344
}
]
}
},
```
**Steps to reproduce**:

1. Send `hostmetrics` data from an OpenTelemetry collector, v0.82.0
2. to apm server v8.9

**Provide logs (if relevant)**: