Generic S3 logs to Loki via lambda-promtail

I want to send Fastly logs stored in S3 to Loki via lambda-promtail. Looks like lambda-promtail can only process predefined types (i.e. CloudTrail, CloudFront, GuardDuty, etc.) of logs. Is there a way to just send generic logs in an S3 bucket to Loki?

Couple of options I can think of:

  1. Write your own parser based on lambda-promtail.
  2. Use a script of some sort to feed your S3 files to Alloy. There are many ways you can do this.