[Suggestion] export SetParentError from the crate

## Feature Request

### Motivation

I have a function with a OpenTelemetrySpanExt::set_parent call. It is naturally to return a Result<_, SetParentError> from my function. But SetParentError is not exported from the tracing-opentelemetry crate. So I can't describe the function signature easily

### Proposal

export SetParentError from the crate, not only OpenTelemetrySpanExt