Tracing configuration API

[lmolkova]

This has started in #261.

• TraceFactory provides builder-like APIs to configure tracing. TracerFactory is OTel concept very similar to LoggerFactory - allows to get tracer for a category.
• you could use it with DI or without
• It also allows configuring the default factory (for libs integration).

Fixes #206, #93

Before

var exporter = new ZipkinTraceExporter(
  new ZipkinTraceExporterOptions() {
    Endpoint = new Uri("https://<zipkin-server:9411>/api/v2/spans"),
    ServiceName = typeof(Program).Assembly.GetName().Name,
  });
var tracer = new TracerFactory(new BatchingSpanProcessor(exporter), TraceConfig.Default)
    .GetTracer("zipkin-test");
// ...

await exporter.ShutdownAsync(CancellationToken.None);

After

using (var tracerFactory = TracerFactory.Create(
    builder => builder.UseZipkin(o =>
        o.ServiceName = "my-service";
        o.Endpoint = new Uri("https://<zipkin-server:9411>/api/v2/spans"))))
{
    var tracer = tracerFactory.GetTracer("zipkin-test");

    // ...
}

[JamesNK]

Looks good! Lots of public method parameters should have null checks but that's picking nits 😄

[austinlparker]

Other than the null checks, I think it might be useful to have an explicit setter for the tracer name, as it's not entirely clear how that gets set right now or what you should use? Overall I think it's an improvement though.

[austinlparker]

Where is the tracer name coming from here?

[austinlparker]

As above, where is tracer named in example?

[austinlparker]

Tracer name?

[lmolkova]

I think it might be useful to have an explicit setter for the tracer name, as it's not entirely clear how that gets set right now or what you should use?

@austinlparker can you elaborate?
The tracer name is part of the named-tracers spec and it's like logging category, so you always request named tracer and factory creates (reuses) one for you.

New Tracer instances can be created via a TracerFactory and its getTracer
method. This method expects two string arguments:

• name (required): This name must identify the instrumentation library (also
  referred to as integration, e.g. io.opentelemetry.contrib.mongodb) and not
  the instrumented library.
  In case an invalid name (null or empty string) is specified, a working
  default Tracer implementation as a fallback is returned rather than returning
  null or throwing an exception.
  A library, implementing the OpenTelemetry API may also ignore this name and
  return a default instance for all calls, if it does not support "named"
  functionality (e.g. an implementation which is not even observability-related).
  A TracerFactory could also return a no-op Tracer here if application owners configure
  the SDK to suppress telemetry produced by this library.
• version (optional): Specifies the version of the instrumentation library
  (e.g. semver:1.0.0).

So, could you help me understand if you want to improve the documentation or mean some API change?

[lmolkova]

Thanks everyone for the review! and special thanks to @JamesNK and @pakrym

Merging this, if anyone has more feedback, I'll address it in another PR.