Add log_schema.kubernetes_key global option

[binarylogic]

Similar to #1823 we need to:

• Add a log_schema.kubernetes_key option. This should
• Use this key in the kubernetes source
• Use this key in the kubernetes_metadata transform
• Use this key when mapping in the loki and datadog sinks. Any others? Should we open these as separate issues?

[LucioFranco]

Looks like we can add the first two in this list today but the rest are blocked on the kubernetes_pod_metadata transform.

[MOZGIII]

I feel like this needs an RFC. We need a way to dynamically extend the schema and refer to the fields in it.

Practically, implementing this PR would be a simple task, yet, I'm a bit hesitant with this approach. As I understand, the main goal would be to automate the interoperation with loki and other sources that follow the same idea of having a semi-manually composed subset of fields that are considered interesting (labels in particular in loki). There are a couple of alternative approaches that seem to be a better fit for that particular task.

I'll elaborate more on this in a dedicated issue.

We can omit this issue from v1 feature set for now.

[binarylogic]

I agree. I think all of the field mapping in sinks is better served with metadata about fields instead of a strict a schema.

[MOZGIII]

Should we close this one?