Refactor handling labels for logging, metrics and debug commands

[diafour]

There is a myriad of map[string]string initialized and passed between layers. The upper-level code should know which labels to pass to the lower-level methods and lower-level has no access to additional information. It seems that Context can help: labels can be stored as context values. Upper-level code just pass a context and lower-level code retrieve everything from this context.

https://github.com/flant/addon-operator/issues/138