Add custom SenzaInfo fields to the Scalyr Agent

[tgrigoryan]

It would be nice to include custom fields specified under the SenzaInfo section in logs as a field/column.

The use case
We have separate senza.yaml files with the same SenzaInfo.StackName per each environment (integration, staging and etc) and we are using SenzaInfo.Environment field to specify the environment of the stack.
Since we get all our logs from all environments in the single place in Scalyr, it would be nice to be able to filter them by Environment field.

SenzaInfo:
  StackName: our-app-name
  Environment: integration