[META 814] Normalize metrics reported based on cgroup in all APM agents

[elastic-apm-tech]

See meta issue and spec for the description and details:

• Meta issue: Normalize metrics reported based on cgroup in all APM agents apm#814
• Spec issue: [META 814] Spec: Normalize metrics reported based on cgroup in all APM agents apm#815

[stevejgordon]

1. Having reviewed this for .NET in Make sure CPU metrics are cgroup aware #2122, we don't feel there is anything further we can do to adjust the CPU metrics for CGroups and doing so would diverge from Microsoft metrics.
2. We already do not send the magic string 9223372036854771712 for system.process.cgroup.memory.mem.limit.bytes. I need to check further to see if we fall back to system.memory.total