fix(lambda): generate correct metrics for aliases

Lambda supports metrics on individual aliases by specifying the metric
dimensions with `FunctionName` set to the underlying function's `Name`,
and `ResourceName` set to the qualified function `Name` (i.e:
`FunctionName:aliasName`).

The previous iteration of this code used the function's ARN instead of
it's name, resulting in incorrect metrics.

Fixes #3724

packages/@aws-cdk/aws-lambda/lib/alias.ts

@@ -156,10 +156,10 @@ export class Alias extends QualifiedFunctionBase implements IAlias {
     return super.metric(metricName, {
       dimensions: {
         FunctionName: this.lambda.functionName,
-        // construct the ARN from the underlying lambda so that alarms on an alias
+        // construct the name from the underlying lambda so that alarms on an alias
         // don't cause a circular dependency with CodeDeploy
         // see: https://github.com/aws/aws-cdk/issues/2231
-        Resource: `${this.lambda.functionArn}:${this.aliasName}`
+        Resource: `${this.lambda.functionName}:${this.aliasName}`
       },
       ...props
     });

packages/@aws-cdk/aws-lambda/test/test.alias.ts

@@ -187,12 +187,7 @@ export = {
           'Fn::Join': [
             '',
             [
-              {
-                "Fn::GetAtt": [
-                  "MyLambdaCCE802FB",
-                  "Arn"
-                ]
-              },
+              { Ref: "MyLambdaCCE802FB" },
               ':prod'
             ]
           ]