OpenTracing shim constructor doesn't match spec

[pjanotti]

Per spec, the operation to create the OpenTracing Tracer shim should receive a TracerProvider and use it to create a Tracer with the name opentracing-shim. Currently, it receives a Tracer and lets the user select the name of the Tracer:

opentelemetry-dotnet/examples/Console/TestOpenTracingShim.cs

Lines 40 to 42 in ce1e196

	var tracer = new TracerShim(
	TracerProvider.Default.GetTracer("MyCompany.MyProduct.MyWebServer"),
	Propagators.DefaultTextMapPropagator);

Of course, removing the current constructor will be a breaking change. Since no stable versions of the OT shim package were released so far it seems that this should be done rather soon. The current constructor could be marked as obsolete for one or two releases before removing it completely.