use router dispatch start events to update name of span

[tsloughter]

Instead of creating the span on Plug.Telemetry events it now uses the plug adapter events and the router dispatch event to update the name to the name of the route.

Depends on this patch to Plug elixir-plug/plug#934 so uses the github repo as the dep until a new release is made.

[garthk]

Now you're relying on [:plug_adapter, :call], reckon you should remove your event_prefix argument.

[tsloughter]

Yea, think I wasn't sure if it would be used for anything else, like adding support for Plug.Telemetry as well. But will remove it until that is added, if ever.

[garthk]

Strikes me you could save yourself a lot of logic if you took a (Conn.t() -> boolean()) | {module(), atom()} filter option to call to find out whether to trace or not.

[garthk]

… though I'm betting people will beg to change the span name, too…

[garthk]

… and also the attributes eg. to handle RFC 7239 Forwarded headers as well as X-Forwarded-For, so by here I'm trying to come up with a clean way of letting them express the whole lot.

Assuming we trust them to propagate — handy if they need to accept other headers! — let's expose methods to determine attributes and to start spans with out tracer, and then call them back?

  # our code
  def handle_start(_, _, %{conn: conn}, %{custom_start: custom_start}) do
    custom_start.(conn)
  rescue
    _ -> :any_plans_for_how_to_stop_one_mistake_from_turning_off_tracing_forever?
  end

  @doc "Call `OpenTelemetry.Tracer.start_span/2` with our tracer, not yours."
  def start_span(span_name, start_opts), do:
    OpenTelemetry.Tracer.start_span(span_name, start_opts)

  # their code
  def my_custom_start(conn, start_span) do
    if my_should_trace?(conn) do
      my_propagate_from(conn.req_headers)
      OpentelemetryPlug.start_span(span_name, %{attributes: my_attributes(conn)})
    end
  end

  defp my_should_trace?(conn) do
    # checks the method or route, whatever
  end

  defp my_propagate_from(conn) do
    # calls :ot_propagation.http_extract/1 if it sees a traceparent
    # ensures equivalent if it sees another header
  end

  defp my_attributes(conn) do
    # calls OpentelemetryPlug.default_attributes/1
    # pipes it through Map.merge/2 with some more attributes
  end

[garthk]

Extracted to #3 so you don't need to ponder it as part of this PR.

[garthk]

elixir-plug/plug#934 is shipped in Plug 1.10.1

[garthk]

Suggested change

	{:plug, github: "elixir-plug/plug", branch: "master"},
	{:plug, "~> 1.10.1"},

[garthk]

Suggested change

"plug": {:git, "https://github.com/elixir-plug/plug.git", "9c880dee5a3d55c81f97a87076e540a9b5878cbd", [branch: "master"]},

"plug": {:hex, :plug, "1.10.1", "c56a6d9da7042d581159bcbaef873ba9d87f15dce85420b0d287bca19f40f9bd", [:mix], [{:mime, "~> 1.0", [hex: :mime, repo: "hexpm", optional: false]}, {:plug_crypto, "~> 1.1.1 or ~> 1.2", [hex: :plug_crypto, repo: "hexpm", optional: false]}, {:telemetry, "~> 0.4", [hex: :telemetry, repo: "hexpm", optional: true]}], "hexpm", "b5cd52259817eb8a31f2454912ba1cff4990bca7811918878091cb2ab9e52cb8"},