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/cloudwatch_integration_test.go

@@ -39,7 +39,7 @@ func Test_CloudWatch_CallResource_Integration_Test(t *testing.T) {
 		return &logApi
 	}
 	ec2Mock := &mocks.EC2Mock{}
-	ec2Mock.On("DescribeRegions", mock.Anything, mock.Anything).Return(&ec2.DescribeRegionsOutput{}, nil)
+	ec2Mock.On("DescribeRegionsWithContext", mock.Anything, mock.Anything).Return(&ec2.DescribeRegionsOutput{}, nil)
 	NewEC2Client = func(provider client.ConfigProvider) models.EC2APIProvider {
 		return ec2Mock
 	}