feat: prefer 'traceparent' header over 'elastic-apm-traceparent' header

[trentm]

The w3c traceparent header is standardized and widely used enough now in Elastic tools that we should prefer it over the proprietary 'elastic-apm-traceparent' header.

See similar for Java, .NET.

Checklist

• Implement code
• Add CHANGELOG.asciidoc entry
• Commit message follows commit guidelines

[apmmachine]

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• Build Cause: Pull request #2079 updated

• Start Time: 2021-05-26T22:17:47.193+0000

• Duration: 19 min 7 sec

• Commit: 95580cd

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Test	Results
Failed	0
Passed	21428
Skipped	0
Total	21428

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[astorm]

Approving pending CI passing.

Just talking through the consequences of this out loud, there's a theoretical change in behavior if a user is setting both of these headers and we start preferring the W3C trace header over the old proprietary header.

However, this doesn't seem like anything that should cause a breaking change -- the behavior change will be better interoperability with the standards based header.

The only users who will see a degradation in their experience are folks who might have some services that add both headers, and other services that only add the old proprietary header. That seems like an edge case on an edge case (and they can always remain on an older agent), so I don't think we'll need a major version change for this change.

[astorm]

jenkins run the tests

[astorm]

jenkins run the tests please