Plan for span event API deprecation

[alexmojaki]

Based on https://github.com/open-telemetry/opentelemetry-specification/pull/4430, the methods span.add_event and span.record_exception will eventually need to be deprecated. Usages of span.record_exception will need to be replaced by logging calls.

I discussed at length with @trask and @lmolkova what this means for SDKs like Python which automatically call span.record_exception within commonly used SDK conveniences like tracer.start_as_current_span. Essentially these will have to be deprecated too on the main Tracer classes since ordinary tracers shouldn't be coupled to the logging API. They can instead be added to specialised tracer classes (see https://github.com/open-telemetry/opentelemetry-js/blob/c6ab2ba4887d0cbbbdf3c1bda229440c67a8497e/api/src/experimental/trace/SugaredTracer.ts for an example) which aren't in the main API/SDK packages. These new packages can make major releases with breaking changes without causing as much disruption as in the core API/SDK packages. It seems that this is what should have been done in the first place since these kinds of conveniences aren't defined in the OTel API spec.