Custom route filter, header propagation, etc

[garthk]

Splitting this one out of #1…

  @doc false
  def handle_start(_, _measurements, %{conn: conn}, _config) do
    # TODO: add config for what paths are traced

Users might want to customise:

• Which paths are traced and which aren't
• What sample rate to apply, either through a kludge like Provide interim sampling capability in exporter garthk/opentelemetry_honeycomb#5 or some later API-blessed solution
• How http.client_ip is derived from headers eg. they need RFC 7239 Forwarded as well as X-Forwarded-For
• How the trace context is derived from headers eg. they need to handle Honeycomb beeline as well as traceparent
• Their own attributes derived from the conn
• Other stuff that hasn't occurred to me

Strikes me we could let them do all that with one config item instead of half a dozen:

• Add a custom_start callback option
• Expose our attributes making machinery so they can call it to get most of the heavy lifting done for free

Our code would look something like this:

  # 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)

Not hard, and lets them get as custom as they want:

  # 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