Forging span links using OpenCensus and Honeycomb

example.ex

  @doc """
  Forge links between spans.

  Preferred, but not supported by our reporter:

      :PARENT_LINKED_SPAN
      |> :oc_span.link(link_trace_id, link_span_id, %{})
      |> :oc_trace.add_link(span_ctx)

  ... but that's ok, [Honeycomb can link using the attributes of extra spans][send-links].

  [send-links]: https://docs.honeycomb.io/getting-data-in/tracing/send-trace-data/#links
  """
  @spec link_span_ctx(
          :undefined | span_ctx(),
          :undefined | span_ctx(),
          :CHILD_LINKED_SPAN | :PARENT_LINKED_SPAN | :TYPE_UNSPECIFIED
        ) :: :ok
  def link_span_ctx(our_span_ctx, their_span_ctx, link_type \\ :PARENT_LINKED_SPAN)

  def link_span_ctx(:undefined, _, _), do: false

  def link_span_ctx(_, :undefined, _), do: false

  def link_span_ctx(our_span_ctx, their_span_ctx, link_type) do
    {:span_ctx, link_trace_id, link_span_id, _, _} = their_span_ctx

    attributes = %{
      "trace.link.span_id" => to_string(:io_lib.format("~16.16.0b", [link_span_id])),
      "trace.link.trace_id" => to_string(:io_lib.format("~32.16.0b", [link_trace_id])),
      "meta.span_type" => "link"
    }

    link_type
    |> Atom.to_string()
    |> start_span(our_span_ctx, attributes: attributes)
    |> finish_span()

    :ok
  end

start_span/3 is a wrapper around :oc_trace.start_span/3; finish_span/1 is a wrapper around :oc_trace.finish_span/1.