fix: print a better message if Step Function context injection is already set up

[lym953]

Problem

#1443 and #1447 introduce a bug that when context injection is already set up for a Lambda or a Step Function in a Step Function, a warning will be printed, saying context injection won't work, which is wrong:

What

If context injection is already set up, i.e.

• a Lambda has "Payload.$": "States.JsonMerge($$, $, false)"
• a Step Function has "CONTEXT.$": "States.JsonMerge($$, $, false)"

then print a better error message:

Context injection is already set up. Skipping context injection.

Testing

Run datadog-ci stepfunctions instrument command twice. The second time, these messages were printed:

Review checklist

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

[datadog-datadog-prod-us1]

Datadog Report

Branch report: yiming.luo/fix-step-func-6
Commit report: 75bfd2a
Test service: datadog-ci-tests

✅ 0 Failed, 376 Passed, 0 Skipped, 1m 27.65s Total duration (2m 0.03s time saved)

[avedmala]

LGTM 🚢

much better user experience!