Remove metadata query when role === "any"

[YousefED]

if role is set to "any", we don't need to query the metadata service explicitly.

node aws-sdk will already use the correct credentials by itself

[trexx]

Thanks, this is very handy.
When using roles, by default AWS access keys and ID's expire after 12 hours. A new key or ID isn't fetched when this happens so this backend will fail to upload metrics to cloudwatch.
aws-sdk will fetch new credentials as they approach expiry.