Lambda-Promtail: add ability to add custom tags to all logs forwarded from Cloudwatch

[JBSchami]

Is your feature request related to a problem? Please describe.
I am looking to use lambda-promtail to send CloudWatch logs to a Loki instance. Everything works, however I would like to be able to add additional custom labels without needing to deploy a promtail instance to EC2 to perform re-labelling before forwarding the logs to my Loki instance.

Describe the solution you'd like
I would like to be able to set additional labels via an environment variable entry (possibly a JSON string?). These additional label values would be added to all logs pushed by the lambda-promtail.

Describe alternatives you've considered
We have considered deploying a Promtail in EC2, with a re-labeling configuration, as described in the docs, but would prefer not to have to do this for the sole purpose of adding additional labels.

Additional context
N/A