Error message must be ignored when status code is ok on Span->SetStatus

[jsuereth]

Right now this behavior is delegated to Recordable (e.g. see https://github.com/open-telemetry/opentelemetry-cpp/blob/main/exporters/otlp/src/otlp_recordable.cc#L293)

The spec states:

An API to set the Status. This SHOULD be called SetStatus. This API takes the StatusCode, and an optional Description, either as individual parameters or as an immutable object encapsulating them, whichever is most appropriate for the language. Description MUST be IGNORED for StatusCode Ok & Unset values.