Add support for cgroup-based CPU metrics

**Is your feature request related to a problem? Please describe.**
CPU metrics reported from containerised services are inaccurate.

**Describe the solution you'd like**
So far we only support [memory](https://github.com/elastic/apm/issues/291) cgroup-based metrics, we need to apply the same for CPU.

***Steps***
- [ ] Define the related data model and intake. @elastic/apm-server your assistance with that, based on Metricbeat and/or stack monitoring, would be great
- [ ] Define how and when UI will be using those
- [ ] Extend the [APM agents spec](https://github.com/elastic/apm/blob/master/specs/agents/metrics.md#cgroup-metrics) accordingly

**Describe alternatives you've considered**
Not supporting those and keep inaccurate 🙂

**Additional context**