use exp backoff on 429 from agent

[ajgajg1134]

What does this PR do?

Motivation

Plugin Checklist

• Unit tests.
• Integration tests.
• Benchmarks.
• TypeScript definitions.
• TypeScript tests.
• API documentation.
• CI jobs/workflows.

Additional Notes

[codecov]

Codecov Report

❌ Patch coverage is 89.09091% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.05%. Comparing base (3ffe266) to head (21ba23d).
⚠️ Report is 6 commits behind head on master.

Files with missing lines	Patch %	Lines
packages/dd-trace/src/exporters/agent/writer.js	89.09%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6784      +/-   ##
==========================================
- Coverage   84.06%   84.05%   -0.01%     
==========================================
  Files         506      506              
  Lines       21211    21295      +84     
==========================================
+ Hits        17830    17900      +70     
- Misses       3381     3395      +14     

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