Review labels consistency in lambda-promtail

[thepalbi]

Lambda promtail adds the __aws_log_type label to entries, to tell apart which was the logging source of that entry. This label is added for s3-based logs, but not for cloudwatch logs for example.

We should review the supported sources, and make these labels consistent.

[spedersen-emailage]

Looks like kinesis and s3 are the only two that add these labels.

https://github.com/search?q=repo%3Agrafana%2Floki%20__aws_log_type&type=code

[cstyan]

a test that enforces that this label is always set seems valid?