fix: use DD_SERVICE for cold start spans instead of hardcoded aws.lambda - #855

• All applicable files are owned properly
  _{Digest: 487ac67}

• At least one approving review from someone other than the latest commit author is required. In the future, it will block merging.
  _{Questions about this check? Reach out in #dx-source-code-management on Slack.
  Digest: 6021e7f}

• All required reviewers must approve this pull request before it can be merged. Learn more in our FAQ.

• Needs approval from Team: apm-serverless

• Needs approval from Team: serverless-aws

  _{Questions about this check? Reach out in #dx-source-code-management on Slack.
  Digest: bd8e1c8}

if this seems incorrect, you can retrigger the evaluation of the MergeGate by clicking on this link