Add OpenTelemetry.Hosting

[JamesNK]

Replaces #253. Now uses the new factory API.

OpenTelemetry.Hosting now does a lot less. Its purpose is registering TracerFactoryBase instance with DI, and starting it with the host.

services.AddOpenTelemetry(builder =>
{
    builder
        .SetSampler(Samplers.AlwaysSample)
        .UseZipkin(o => o.ServiceName = "my-service")

        // you may also configure request and dependencies collectors
        .AddRequestCollector()
        .AddDependencyCollector())
});

TelemetryFactoryHostedService creates the factory when the host is started. If the factory and its collectors can be created immediately in ConfigureService then TelemetryFactoryHostedService could be removed.

[JamesNK]

Usually you would use the builder overload. Because this sample completely re-implements the factory, it and the collectors have to be created manually.

[lmolkova]

Looks awesome!

[lmolkova]

Let's keep it open till Monday so everyone has a chance to look

[lmolkova]

@JamesNK if you don't mind, could you please resolve some merge conflicts? We also moved usings outside of namespace and new files might need to be changed too.

[bruno-garcia]

LGTM. One nit is that unfortunately the overloads with Func<IServiceProvider, TraceFactoryBase> are not covered by tests

[JamesNK]

I'm at NDC Sydney this week. I should find some time to look at it, I just don't know when.

[lmolkova]

@JamesNK no problem, take your time

[SergeyKanzhelev]

@JamesNK do you mind if I'd merge for you?

[JamesNK]

Be my guest.

I was planning to add xml docs to the new APIs, but that can be in a later PR.

[JamesNK]

Merge pls 🙏