RUMM-675 Add tracer.startRootSpan() API

[ncreated]

What and why?

📦 This PR adds the startRootSpan() API to Datadog.Tracer.

This API enables using setActive() span in asynchronous jobs, e.g.:

func makeAPIRequest(on queue: DispatchQueue, completion: @escaping () -> Void) {
    let requestSpan = tracer
        .startRootSpan(operationName: "request")
        .setActive() // set active, so the asynchronous job can continue the trace without the span reference

    queue.asyncAfter(deadline: .now() + 1) {
        let responseDecodingSpan = tracer.startSpan(operationName: "response decoding")
        responseDecodingSpan.finish()
        requestSpan.finish()
        completion()
    }
}

makeAPIRequest(on: queue) {}
makeAPIRequest(on: queue) {}

which results with two root "request" spans, each having "response decoding" child span.

How?

New API was introduced:

/// Start a new root span with the given operation name.
/// - Parameters:
///   - operationName: the operation name for the newly-started span
///   - tags:          a set of tag keys and values per `OTSpan#setTag:value:`, or `nil` to start with
///                    an empty tag map
///   - startTime:     an explicitly specified start timestamp for the `OTSpan`, or `nil` to use the
///                    current walltime
/// - returns:         a valid Span instance; it is the caller's responsibility to call `finish()`.
func startRootSpan(
    operationName: String,
    tags: [String: Encodable]?,
    startTime: Date?
) -> OTSpan

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

[buranmert]

1. setActive() can be called both on normal and root spans, correct?
2. are we gonna add this to the OpenTracing/Telemetry API (the original repo) as well?

[ncreated]

• setActive() can be called both on normal and root spans, correct?

Yes. Regular span can become the active span in the current execution context. Same for the root span.

• are we gonna add this to the OpenTracing/Telemetry API (the original repo) as well?

Not for OT, as there's no activity in opentracing-swift for many months which is why we decided to put the OT code in our repo. Not sure if there's startRootSpan() concept in OTel - @nachoBonafonte ?

[nachoBonafonte]

In OTel, there is a span builder, and it has a setNoParent() method, that basically makes a span a root span when is built