Skip sfn->Lambda context injection of Payload is not object (case 3)

What

In a State Machine, if a Lambda execution step's Payload field is not a JSON object, e.g.

    "Lambda Invoke": {
      "Type": "Task",
      "Resource": "arn:aws:states:::lambda:invoke",
      "OutputPath": "$.Payload",
      "Parameters": {
        "FunctionName": "arn:aws:lambda:sa-east-1:425362996713:function:yimingPlayground",
        "Payload": "Just a string!"
      },
      "Next": "Step Functions StartExecution"
    },

then print a warning and skip context injection.

This is case # 3 in this design doc: Fixing Step Function Instrumentation

Why

To inject context into Lambda payload, the payload must be a JSON dictionary. Injection can't work if payload is a string.

Testing

Automated testing

Passed the added test and existing tests

Manual testing

Steps

Change the Lambda Execution step to have a string Payload field as shown in the "What" section
Build the package: yarn prepack
Instrument the Step Function using ./dist/cli.js stepfunctions instrument

Result

A warning is printed:

Review checklist

[x] Feature or bugfix MUST have appropriate tests (unit, integration)

DataDog / datadog-ci

Skip sfn->Lambda context injection of Payload is not object (case 3) #1461