feat(stepfunctions): ExecutionTime metric (#2498)

The `ExecutionTime` metric was not supported by the L2 `StateMachine` construct.
This PR adds a `metricTime()` function to `StateMachine`.

See https://docs.aws.amazon.com/step-functions/latest/dg/procedure-cw-metrics.html#cloudwatch-step-functions-execution-metrics

Author: Simon-Pierre Gingras

Diff for: packages/@aws-cdk/aws-stepfunctions/lib/state-machine.ts

@@ -193,6 +193,15 @@ export class StateMachine extends Resource implements IStateMachine, events.IEve
     public metricStarted(props?: cloudwatch.MetricOptions): cloudwatch.Metric {
         return this.metric('ExecutionsStarted', props);
     }
+
+    /**
+     * Metric for the interval, in milliseconds, between the time the execution starts and the time it closes
+     *
+     * @default sum over 5 minutes
+     */
+    public metricTime(props?: cloudwatch.MetricOptions): cloudwatch.Metric {
+        return this.metric('ExecutionTime', props);
+    }
 }
 
 /**