Optionally use IAM authentication

[mcanevet]

It would be great to be able to use IAM authentication when the exporter runs on AWS: https://docs.aws.amazon.com/AmazonRDS/latest/AuroraUserGuide/UsingWithRDS.IAMDBAuth.Connecting.Go.html

[tejavegesna-lb]

Agree with @mcanevet We need a way to use IAM Authentication for metrics

[lasith-kg]

Agree! Support for dynamic credentials (of the likes of RDS IAM Auth) would be great from a security posture

[bvalente]

@mcanevet @tejavegesna-lb @lasith-kg created this PR 🙂 #983