Avoid Activity.SetCustomPropery usage for Resource.

[cijothomas]

Currently TracerProviderSdk stores Resource inside every Activity. https://github.com/open-telemetry/opentelemetry-dotnet/blob/master/src/OpenTelemetry/Trace/TracerProviderSdk.cs#L81
Internally, resource is stored inside Activity.SetCustomProperty. Perf benchmark shows that the 1st time CustomProperty is used, we pay a big price, with subsequent ones having no impact. We already modified all instrumentations to not user CustomProperty. Now the only reason for using CustomProperty is to store Resource.

Proposing to avoid CustomProperty by moving Resource elsewhere. Options include:

1. Store resource inside activity.TagObject (it now support string,obj, so feasible to do this). This ensures every span/activity will be associated with a resource, as required by the spec.

2. Dont store Resource anywhere inside the Activity itself. Instead pass it to all the Processors. TracerProviderSdk knows about Resource and about Processors. So it can set the Resource to the Processors. We may introduce some new interface like ISetResource(resoiurce), and for those processors which implements this, have TracerProviderSdk call it and set the resource.
   This ensures processor/exporter has a way to get the resource associated, but without actually having to store it in every activity. This should be a good win for perf.

3. Similar to 2, introduce an interface ISetProvider that can be used to pass provider (TracerProvider, MeterProvider, LogProvider, etc.) to processors, exporters, etc.

Happy to hear more ideas. This was based on discussions with @eddynaka

[cijothomas]

My preference is to store Resource inside TagObject, just like Status. We can use the keyname as "otel.resource" (status uses otel.status_code). We need to provide general guidance that any tags with reserved prefix "otel.", should not be treated as regular tags.