CGroup memory utilization metric in stack monitoring for integration server is not available on ESS

**APM Server version** (`apm-server version`): 8.3.*

**Description of the problem including expected versus actual behavior**: Stack monitoring should show memory utilization for integration server

**Steps to reproduce**:

1. On ESS, open stack monitoring
2. Open `Integrations server overview`
3. Observe memory panel in `Integrations Server - Resource Usage`

**Other details**

The metric seems to plot `beats_stats.metrics.beat.cgroup.memory.mem.usage.bytes` but as per [metricbeat documents](https://www.elastic.co/guide/en/beats/metricbeat/8.0/exported-fields-beat.html) the correct field should be either `beats_stats.metrics.beat.cgroup.mem.usage.bytes` or `beat.stats.cgroup.memory.mem.usage.bytes`