Cloudwatch: Fix Unexpected error (#74420)

Fix unexpected error when creating a new cloudwatch datasource.

Involves a fair amount of refactoring, so if this causes unexpected issues related to region fetching we can turn this off with the cloudwatchNewRegionsHandler feature toggle, although we do not predict it will so we are enabling it to default to true and hope to remove it shortly.

pkg/tsdb/cloudwatch/client_factory.go

@@ -49,10 +49,10 @@ var NewCWLogsClient = func(sess *session.Session) cloudwatchlogsiface.CloudWatch
 	return cloudwatchlogs.New(sess)
 }
 
-// EC2 client factory.
+// NewEC2Client is a client factory.
 //
 // Stubbable by tests.
-var newEC2Client = func(provider client.ConfigProvider) models.EC2APIProvider {
+var NewEC2Client = func(provider client.ConfigProvider) models.EC2APIProvider {
 	return ec2.New(provider)
 }