fix(step-functions): Infer construct fallback name from Lambda function (#407)

Fixes #274

Use the same strategy that [we use for vanilla
Lambda](https://github.com/cdklabs/cdk-monitoring-constructs/blob/cec84dba29fa60d55fcbc23ae71d13c32246182d/lib/monitoring/aws-lambda/LambdaFunctionMonitoring.ts#L169).

---

_By submitting this pull request, I confirm that my contribution is made
under the terms of the Apache-2.0 license_

Author: Eugene Cheung

lib/monitoring/aws-step-functions/StepFunctionLambdaIntegrationMonitoring.ts

@@ -3,6 +3,7 @@ import {
   HorizontalAnnotation,
   IWidget,
 } from "aws-cdk-lib/aws-cloudwatch";
+import { IFunction, CfnFunction } from "aws-cdk-lib/aws-lambda";
 
 import {
   StepFunctionLambdaIntegrationMetricFactory,
@@ -71,10 +72,10 @@ export class StepFunctionLambdaIntegrationMonitoring extends Monitoring {
   ) {
     super(scope, props);
 
-    const fallbackConstructName = props.lambdaFunction.functionName;
     const namingStrategy = new MonitoringNamingStrategy({
       ...props,
-      fallbackConstructName,
+      namedConstruct: props.lambdaFunction,
+      fallbackConstructName: this.resolveFunctionName(props.lambdaFunction),
     });
     this.title = namingStrategy.resolveHumanReadableName();
     this.functionUrl = scope
@@ -260,4 +261,9 @@ export class StepFunctionLambdaIntegrationMonitoring extends Monitoring {
       }),
     ];
   }
+
+  private resolveFunctionName(lambdaFunction: IFunction): string | undefined {
+    // try to take the name (if specified) instead of token
+    return (lambdaFunction.node.defaultChild as CfnFunction)?.functionName;
+  }
 }