CloudWatch: Use context in aws DescribeRegionsWithContext (#76922)

* Use context in aws DescribeRegionsWithContext

In the current way, DescribeRegions is used which doesn't allow
cancelling the request if the context changes. Using
DescribeRegionsWithContext is the preferred way.

* Fix context variable

* Revert GetRegionsWithContext to GetRegions

GetRegions is not an AWS SDK method. Hence, GetRegions should be enough
as the name change is not needed for context implementation.

pkg/tsdb/cloudwatch/mocks/regions.go

@@ -1,6 +1,8 @@
 package mocks
 
 import (
+	"context"
+
 	"github.com/aws/aws-sdk-go/aws"
 	"github.com/aws/aws-sdk-go/aws/request"
 	"github.com/aws/aws-sdk-go/service/ec2"
@@ -12,7 +14,7 @@ type RegionsService struct {
 	mock.Mock
 }
 
-func (r *RegionsService) GetRegions() ([]resources.ResourceResponse[resources.Region], error) {
+func (r *RegionsService) GetRegions(ctx context.Context) (in []resources.ResourceResponse[resources.Region], e error) {
 	args := r.Called()
 	return args.Get(0).(([]resources.ResourceResponse[resources.Region])), args.Error(1)
 }
@@ -21,7 +23,7 @@ type EC2Mock struct {
 	mock.Mock
 }
 
-func (e *EC2Mock) DescribeRegions(in *ec2.DescribeRegionsInput) (*ec2.DescribeRegionsOutput, error) {
+func (e *EC2Mock) DescribeRegionsWithContext(ctx aws.Context, in *ec2.DescribeRegionsInput, opts ...request.Option) (*ec2.DescribeRegionsOutput, error) {
 	args := e.Called()
 	return args.Get(0).(*ec2.DescribeRegionsOutput), args.Error(1)
 }