[agents] Read k8s metadata from the /proc filesystem

[roncohen]

Similar to #22, it turns out you can in most circumstances read the k8s metadata from inside the container.

This issue follows on the work in #21 and is about reading the data automatically, so users don't need to use the Downward API. If environment variables are set, they should take precedence.

Please see @axw's comment for the implementation details: #21 (comment)

@elastic/apm-agent-devs Please link your issue or PR and then mark the checkbox when the implementation is merged:

• - Java Read k8s metadata from the /proc filesystem apm-agent-java#383
• - Python added parsing of cgroups file apm-agent-python#352
• - Ruby Read k8s metadata from the /proc filesystem apm-agent-ruby#274
• - Go - Add Kubernetes metadata apm-agent-go#342
• - dotNet - Report kubernetes system metadata apm-agent-dotnet#181
• - Node.js - Read k8s metadata from the /proc filesystem apm-agent-nodejs#737
• - Documentation - Docs: add kubernetes metadata apm-server#1650

[axw]

Closing this as all agents apart from .NET have implemented, and .NET has an issue to track it. In addition, the logic is captured in https://github.com/elastic/apm/blob/master/docs/agent-development.md#containerkubernetes-metadata now.