feat: fix resource autodetection for GKE, GAE, and GCE

[0xSage]

Fixes #673 🦕
Fixes #125 🦕

For GKE, we infer the following after thorough discussion and research:

• pod_name == process.env.HOSTNAME
• namespace_name == fs/readfile() ~/var/run/secrets/kubernetes.io/serviceaccount/namespace
• container_name is only populated if users pass it into containers as an environment variable. Not available via any downwards APIs or GCP apis for now.

Passing e2e environment tests that verify logging behavior in respective platforms:

• prod:cloud-devrel/client-libraries/go/google-cloud-go/environment/logging/compute
• prod:cloud-devrel/client-libraries/go/google-cloud-go/environment/logging/appengine_standard
• prod:cloud-devrel/client-libraries/go/google-cloud-go/environment/logging/kubernetes

[generated-files-bot]

Warning: This pull request is touching the following templated files:

• .kokoro/environment/appengine_standard.cfg - .kokoro files are templated and should be updated in synthtool
• .kokoro/environment/compute.cfg - .kokoro files are templated and should be updated in synthtool
• .kokoro/environment/kubernetes.cfg - .kokoro files are templated and should be updated in synthtool

[sebtrif]

Heya!

Would it be possible to mention in the docs somewhere or in the readme the gotcha about CONTAINER_NAME needing to be passed via environment variables?

It's a super useful PR but I was left scratching my head for a few hours and it took looking through the source to figure out why that field alone wasn't populated :/

Thanks again for the useful PR, it makes everything combine really nicely with Cloud Monitoring since the changes!