CloudWatch: Refactor around handleGetRegions (#65713)

* Create minimal EC2 interface

* convert regions variable to a function returning a copy

* Add test for multiple calls to regions to check regionsCache

* Add returns to handler after error

* Refactor handleGetRegions

pkg/tsdb/cloudwatch/models/api.go

@@ -5,6 +5,7 @@ import (
 
 	"github.com/aws/aws-sdk-go/service/cloudwatch"
 	"github.com/aws/aws-sdk-go/service/cloudwatchlogs"
+	"github.com/aws/aws-sdk-go/service/ec2"
 	"github.com/aws/aws-sdk-go/service/oam"
 	"github.com/grafana/grafana-plugin-sdk-go/backend"
 	"github.com/grafana/grafana/pkg/services/featuremgmt"
@@ -58,3 +59,8 @@ type OAMAPIProvider interface {
 	ListSinks(*oam.ListSinksInput) (*oam.ListSinksOutput, error)
 	ListAttachedLinks(*oam.ListAttachedLinksInput) (*oam.ListAttachedLinksOutput, error)
 }
+
+type EC2APIProvider interface {
+	DescribeRegions(in *ec2.DescribeRegionsInput) (*ec2.DescribeRegionsOutput, error)
+	DescribeInstancesPages(in *ec2.DescribeInstancesInput, fn func(*ec2.DescribeInstancesOutput, bool) bool) error
+}