Print warning if traces merging from sfn to sfn is skipped

[lym953]

Why?

Let's say Step Function A executes Step Function B. If B:

1. doesn't have a Parameters field, or
2. its Parameters.Input field is not a JSON object
3. its Parameters.Input field has a custom CONTEXT.$ field

traces merging will be skipped, but no warning message will be printed.

What

Print a warning message in these cases to let users know what won't work.

Testing

1. Missing Parameters field

I wasn't able to test this because Parameters field is required when editing the State Machine from AWS Management Console.

2. Parameters.Input field is not a JSON object

Steps

1. Change the Step Function execution step to be like:

   "Step Functions StartExecution": {
     "Type": "Task",
     "Resource": "arn:aws:states:::states:startExecution",
     "Parameters": {
       "StateMachineArn": "...",
       "Input": "Input!"
     },
     "End": true
   }

2. Run datadog-ci stepfunctions instrument command

Result

A warning message is printed as expected:

3. custom CONTEXT.$

4. Happy case

datadog-ci stepfunctions instrument finishes with no error

Review checklist

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

[datadog-datadog-prod-us1[bot]]

Datadog Report

Branch report: yiming.luo/fix-step-func-2 Commit report: 5ef1845 Test service: datadog-ci-tests

:white_check_mark: 0 Failed, 364 Passed, 0 Skipped, 1m 26.6s Total duration (2m 1.5s time saved)

[lym953]

I found a bug: if traces merging is already set up and instrumentation command is run again, the warning will also be printed, which is wrong. I will fix this in a separate PR.