fix: Log correct tracing headers for Datadog debugging

[timmc-edx]

• traceparent is the correct header, not tracecontext.
• Use _header suffix on two of these to avoid collisions with other span tags that Datadog may already be setting in other ways.

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Make sure that the following steps are done before merging:

• Have a Site Reliability Engineer review the PR if you don't own all of the services impacted.
• If you are adding any new default values that need to be overridden when this change goes live, update internal repos and add an entry to the top of the CHANGELOG.
• Performed the appropriate testing.