RUM-1836 feat(otel-tracer): add support for span links

[ganeshnj]

What and why?

Span link is an otel concept which allows casual relationship among spans. This is similar to parent but it can be many-many.

How?

In datadog, span links are supported via _dd.span_links tag which takes a list of links.

A link is represented with

 {
     "trace_id": "<exactly 32 character, zero-padded lower-case hexadecimal encoded trace id>",
     "span_id": "<exactly 16 character, zero-padded lower-case hexadecimal encoded span id>",
     "attributes": {"key":"value", "pairs":"of", "arbitrary":"values"},
     "dropped_attributes_count": <decimal 64 bit integer>,
     "tracestate": "a tracestate as defined in the W3C standard",
     "flags": <an integer representing the flags as defined in the W3C standard>
 }

Because RUM backend only supports [String: String] tags, this implementation escapes the span links JSON to make it a string value.

This

let tracer = Tracer.shared()
let span1 = tracer.spanBuilder(spanName: "span1").setSpanKind(spanKind: .client).startSpan()
span1.end()

let span2 = tracer.spanBuilder(spanName: "span2").setSpanKind(spanKind: .client).startSpan()
span2.end()

let spanBuilder = tracer.spanBuilder(spanName: "span3").setSpanKind(spanKind: .client)
spanBuilder.addLink(spanContext: span1.context)
spanBuilder.addLink(spanContext: span2.context)
let span3 = spanBuilder.startSpan()
span3.end()

results in

We are also using lower 64 bits for trace id as the SDK doesn't support 128 bit ids yet.

Review checklist

• Feature or bugfix MUST have appropriate tests (unit, integration)
• Make sure each commit and the PR mention the Issue number or JIRA reference
• Add CHANGELOG entry for user facing changes

Custom CI job configuration (optional)

• Run unit tests for Core, RUM, Trace, Logs, CR and WVT
• Run unit tests for Session Replay
• Run integration tests
• Run smoke tests
• Run tests for tools/

[datadog-datadog-prod-us1]

Datadog Report

Branch report: ganeshnj/feat/otel-tracer-span-links
Commit report: 8ce2edf
Test service: dd-sdk-ios

✅ 0 Failed, 2753 Passed, 0 Skipped, 11m 36.73s Wall Time
❄️ 2 New Flaky
🔻 Test Sessions change in coverage: 8 decreased, 6 increased

New Flaky Tests (2)

• testIfIPv6AddressIsPrivate - KronosInternetAddressTests - Last Failure

  Expand for error

   Assertion Failure at KronosInternetAddressTests.swift:67: XCTAssertFalse failed - ff:b2f:bb94:fd44:4f9:75a6:4d80:1ae5 should not be private IP
  

• testGivenDelegateSubclass_whenInterceptingRequests_itDetectFirstPartyHost - NetworkInstrumentationFeatureTests - Last Failure

  Expand for error

   Abort trap. Check error.crash_log for the full crash log.
  

🔻 Code Coverage Decreases vs Default Branch (8)

This report shows up to 5 code coverage decreases.

• test DatadogCoreTests tvOS 75.28% (Δ-4.29%) - Details
• test DatadogTraceTests iOS 45.57% (Δ-3.63%) - Details
• test DatadogTraceTests tvOS 45.71% (Δ-3.59%) - Details
• test DatadogCoreTests iOS 69.13% (Δ-3.32%) - Details
• test DatadogInternalTests iOS 79.61% (Δ-1.61%) - Details

[maciejburda]

LGTM