code: func SpanIDFunc(f IDFunc) TracerOption {
  	return func(o *tracerOptions) *tracerOptions {
  		if f == nil {
  			panic("span ID function cannot be nil")
  		}
  		o.spanIDFunc = f
  		return o
  	}
  }
docs: // SpanIDFunc is a constructor option that overrides the function used to
  // compute span IDs.