AWS Signature Version 4 support

[mtanda]

I want to scrape metrics from AWS API Gateway or S3.

To support this safely, I need to add auth layer in front of these service.
I want to use EC2 IAM role for auth request, so I want to add AWS sign v4 support.
http://docs.aws.amazon.com/general/latest/gr/signature-version-4.html

Using external proxy is one of the workaround, but I don't want to launch such proxy per scrape endpoint.
https://github.com/coreos/aws-auth-proxy

[brian-brazil]

This is a non-standard authentication mechanism specific to one provider, I think a proxy is the way to go here.