Summary of changes

Introduce an API to allow the extraction of the SpanContext when the Tracer couldn't do it.

Api:

ISpanContext? Extract<TCarrier>(TCarrier carrier, Func<TCarrier, string, IEnumerable<string?>> getter)

Calling code:

var spanContextExtractor = new SpanContextExtractor();
var parentContext = spanContextExtractor.Extract(headers, (headers, key) => GetHeaderValues(headers, key));
var spanCreationSettings = new SpanCreationSettings() { Parent = parentContext }; 
using var scope = Tracer.Instance.StartActive("operation", spanCreationSettings);

Where GetHeaderValues will depend on the underlying system used. Here are a few examples:

// Confluent.Kafka
IEnumerable<string> GetHeaderValues(Headers headers, string name)
{
    if (headers.TryGetLastBytes(name, out var bytes))
    {
        try
        {
            return new[] { Encoding.UTF8.GetString(bytes) };
        }
        catch (Exception)
        {
            // ignored
        }
    }

    return Enumerable.Empty<string>();
}

// RabbitMQ
IEnumerable<string> GetHeaderValues(IDictionary<string, object> headers, string name)
{
    if (headers.TryGetValue(name, out object value) && value is byte[] bytes)
    {
        return new[] { Encoding.UTF8.GetString(bytes) };
    }

    return Enumerable.Empty<string>();
}

Reason for change

In some cases, propagation cannot happen. For instance, Streamiz, a library implemented above the Confluent Kafka, enqueues messages before spans are created, and thus before context can be propagated.
This API gives the liberty of extracting such context and passing it to custom spans.

Implementation details

A new API called SpanContextExtractor calls our SpanContextPropagator given a carrier and a getter. It is basically a public API above an internal one.

I've chosen to make it non static and to add an interface as some of our customers ask to be given ways to mock our APIs. It is located in Datadog.Trace namespace for better usability for users.
We had discussed other ways to provide this API, but this one seemed the most simple and relevant.

Test coverage

No Utests as it would be testing what we already do.
Changed our RabbitMq tests to validate that distributed tracing is working in both cases

Other details

Fixes APMS-7021