Enable logging in json format

[grzesuav]

GoogleCloudPlatform/metacontroller#167

[AmitKumarDas]

What is the end goal in mind here @grzesuav ?
Do we think exposing operational stuff as metrics achieves the same goal?
Since metac can be used as a library, does it mean all those projects should also use json based logging?

[grzesuav]

To make it compatible with elasticsearch or any distributed logging aggregation system.

It should be configurable, I do not have much insights in go ecosystem.

In java world I would :

• use logging abstraction (like slf4j)
• separate standalone and library packages - like distribution == library + runtime implementation
  so it allow to have common core and let implementation of used logging library up to user and put some additional layer for distribution