feat: implement Span Links API

[kruskall]

This is an attempt to implement the Span Links API, mostly inspired by other PRs to the other agents and the spec/docs for OT.

Closes #1243

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• Start Time: 2022-07-21T20:07:55.652+0000

• Duration: 123 min 55 sec

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Passed	6420
Skipped	148
Total	6568

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🌐 Coverage report

Name	Metrics % (covered/total)	Diff
Packages	100.0% (59/59)	💚
Files	99.346% (152/153)	👍
Classes	96.264% (335/348)	👍 0.011
Methods	90.265% (955/1058)	👍 0.009
Lines	82.178% (11076/13478)	👎 -0.062
Conditionals	100.0% (0/0)	💚

[axw]

Looking good!

We'll need to add this for transactions too.

Can you please also add tests (one for spans, one for transactions)? We should show that the span links are included in the events sent, which we can verify using the apmtest.RecordingTracer. See span_test.go and transaction_test.go for examples. Since the spec says "Links SHOULD preserve the order in which they are set", it would be good to show that in the tests too.

[kruskall]

Thank you for the feedback! 🙇

I've pushed some changes and added the tests.

We'll need to add this for transactions too.

Thank you for pointing this out, I reread the spec and I completely missed that part 🤦

[axw]

Thanks, looks good. I'd like to see the tests simplified, but otherwise LGTM.

[kruskall]

Thanks for the feedback! I've pushed some changes 🏓

[axw]

LGTM! Just a suggestion about the test, I'll leave it to you to decide which way you prefer.

[kruskall]

/test

[kruskall]

/test