Discuss: public API surface and interface usage

[lmolkova]

C# and .NET programming styles do not typically involve interface-for-everything approach.

We should have a discussion about it in opencensus and during separation of API and Impl (#8) ensure that interfaces are not overused

[lmolkova]

E.g. ITraceComponent

    public interface ITraceComponent
    {
        ITracer Tracer { get; }
        IPropagationComponent PropagationComponent { get; }
        IClock Clock { get; }
        IExportComponent ExportComponent { get; }
        ITraceConfig TraceConfig { get; }
    }

    public abstract class TraceComponentBase : ITraceComponent
    {
        internal static ITraceComponent NewNoopTraceComponent => new NoopTraceComponent();
        public abstract ITracer Tracer { get; }
        public abstract IPropagationComponent PropagationComponent { get; }
        public abstract IClock Clock { get; }
        public abstract IExportComponent ExportComponent { get; }
        public abstract ITraceConfig TraceConfig { get; }
    }

There is no visible benefit in having both of them and interface could be removed. For testing purpose virtual methods are as mockable as interfaces.

There could be a case when base class is heavy and requires some additional assembly references - in this case it makes sense to provide an interface

[SergeyKanzhelev]

This will be handled in OpenTelemetry project