def set_baggage_item(key, value)
      @context = SpanContext.new(
        id: context.id,
        trace_id: context.trace_id,
        baggage: context.baggage.merge({key => value})
      )
      self
    end
def set_baggage_item(key, value)
      @context = SpanContext.new(
        id: context.id,
        trace_id: context.trace_id,
        baggage: context.baggage.merge({key => value})
      )
      self
    end
TODO(ngauthier@gmail.com) baggage keys have a restricted format according
 to the spec: http://opentracing.io/documentation/pages/spec#baggage-vs-span-tags
 Set a baggage item on the span
 @param key [String] the key of the baggage item
 @param value [String] the value of the baggage item