Cross account describe ASGs

[amanya]

The motivation of this patch is to make Eureka capable of knowing the state of auto scaling groups even if they are from microservices running in accounts other than the one where Eureka is running.

In that case, it will do a a cross-account assume role to get permission to describe the ASGs.

Here is a walkthrough from Amazon that explains how to setup the cross-account IAM role delegation:

The role must be called ListAutoScalingGroups and the policy must be like this:

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Action": [
                "autoscaling:DescribeAutoScalingGroups"
            ],
            "Effect": "Allow",
            "Resource": "*"
        }
    ]
} 

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[qiangdavidliu]

The accountId metadata field was added to eureka-client relatively recently, so for backwards compatibility, it is possible in existing deployments to have older clients that do not register this field. This will return null here and cause an NPE upstream. Can you please add a null check here and let it fall through instead?

[qiangdavidliu]

@amanya thank you for the pull request, this would be a very useful feature for Eureka to have. I added a couple of minor comments that I hope you can take a quick look at.

Additionally, have you been able to verify this code change? We have mockito as a dependency and it should be able to write a quick unit test for this mocking out the AWS SDK with mockito.

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[amanya]

@qiangdavidliu thanks for your comments :)

I think I've managed to solve the first two issues, but for making the tests, I'll need to use a tool like Powermockito due to the static dependencies of the AwsAsgUtil class, right?

[qiangdavidliu]

Thanks @amanya . We can merge this in and I'll take a look at what kind of testing we can do for this.