teralytics/prometheus-ecs-discovery: main.go; 28 LoC

Found a possible issue in [teralytics/prometheus-ecs-discovery](https://www.github.com/teralytics/prometheus-ecs-discovery) at [main.go](https://github.com/teralytics/prometheus-ecs-discovery/blob/8b048f63b5f042fc04aabf2cb899d200825adebc/main.go#L435-L462)

Below is the message reported by the analyzer for this snippet of code. Beware that the analyzer only reports the first issue it finds, so please do not limit your consideration to the contents of the below message.

> reference to clusterArn was used in a composite literal at line 444

[Click here to see the code in its original context.](https://github.com/teralytics/prometheus-ecs-discovery/blob/8b048f63b5f042fc04aabf2cb899d200825adebc/main.go#L435-L462)

Click here to show the 28 line(s) of Go which triggered the analyzer.

```go
for clusterArn, containerInstancesArns := range clusterArnToContainerInstancesArns {
keys := make([]string, 0, len(containerInstancesArns))
for k := range containerInstancesArns {
keys = append(keys, k)
}

splittedKeys := SplitArray(keys, 100)
for _, chunkedKeys := range splittedKeys {
input := &ecs.DescribeContainerInstancesInput{
Cluster: &clusterArn,
ContainerInstances: chunkedKeys,
}
req := svc.DescribeContainerInstancesRequest(input)
output, err := req.Send()
if err != nil {
return nil, err
}

if len(output.Failures) > 0 {
log.Printf("Described %d failures in cluster %s", len(output.Failures), clusterArn)
}
for _, ci := range output.ContainerInstances {
cInst := ci
clusterArnToContainerInstancesArns[clusterArn][*cInst.ContainerInstanceArn] = &cInst
instanceIDToEC2Instance[*cInst.Ec2InstanceId] = nil
}
}
}

```

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commit ID: 8b048f63b5f042fc04aabf2cb899d200825adebc