Telemetry metadata doesn't have data about response in case of error

[fuelen]

When a middleware returns an error, then metadata.env struct doesn't have status, body, headers etc, while these fields were available in the middleware. I'd expect to have this data in telemetry handler, as it helps in debugging.

Script:

Mix.install([:tesla, :telemetry])

:telemetry.attach_many(
  "tesla.request",
  [[:tesla, :request, :stop]],
  fn _event, measurements, metadata, _ ->
    dbg(metadata)
  end,
  nil
)

defmodule ExpectOKMiddleware do
  @behaviour Tesla.Middleware

  @impl Tesla.Middleware
  def call(env, next, _opts) do
    with {:ok, env} <- Tesla.run(env, next) do
      if dbg(env).status == 200 do
        {:ok, env}
      else
        {:error, {:unexpected_response, "not ok :("}}
      end
    end
  end
end

client = Tesla.client([Tesla.Middleware.Telemetry, {Tesla.Middleware.BaseUrl, "https://hex.pm/api"}, ExpectOKMiddleware])
client |> Tesla.get("/packages/tesla") |> dbg()

output:

[tesla_test.exs:18: ExpectOKMiddleware.call/3]
env #=> %Tesla.Env{
  method: :get,
  url: "https://hex.pm/api/packages/tesla",
  query: [],
  headers: [
    {"cache-control", "max-age=0, private, must-revalidate"},
    {"date", "Fri, 24 Nov 2023 14:41:30 GMT"},
    {"via", "1.1 google"},
    {"server", "Cowboy"},
    {"vary", "accept-encoding"},
    {"content-length", "56"},
    {"content-type", "application/vnd.hex+json; charset=utf-8"},
    {"strict-transport-security", "max-age=31536000"},
    {"x-request-id", "F5qWJEjGVwCUrFsJ5ONB"},
    {"alt-svc", "h3=\":443\"; ma=2592000,h3-29=\":443\"; ma=2592000"}
  ],
  body: "{\"message\":\"User-Agent header is required\",\"status\":400}",
  status: 400,
  opts: [],
  __module__: Tesla,
  __client__: %Tesla.Client{
    fun: nil,
    pre: [
      {Tesla.Middleware.Telemetry, :call, [[]]},
      {Tesla.Middleware.BaseUrl, :call, ["https://hex.pm/api"]},
      {ExpectOKMiddleware, :call, [[]]}
    ],
    post: [],
    adapter: nil
  }
}

[tesla_test.exs:7: (file)]
metadata #=> %{
  env: %Tesla.Env{
    method: :get,
    url: "/packages/tesla",
    query: [],
    headers: [],
    body: nil,
    status: nil,
    opts: [],
    __module__: Tesla,
    __client__: %Tesla.Client{
      fun: nil,
      pre: [
        {Tesla.Middleware.Telemetry, :call, [[]]},
        {Tesla.Middleware.BaseUrl, :call, ["https://hex.pm/api"]},
        {ExpectOKMiddleware, :call, [[]]}
      ],
      post: [],
      adapter: nil
    }
  },
  error: {:unexpected_response, "not ok :("}
}

[tesla_test.exs:28: (file)]
client #=> %Tesla.Client{
  fun: nil,
  pre: [
    {Tesla.Middleware.Telemetry, :call, [[]]},
    {Tesla.Middleware.BaseUrl, :call, ["https://hex.pm/api"]},
    {ExpectOKMiddleware, :call, [[]]}
  ],
  post: [],
  adapter: nil
}
|> Tesla.get("/packages/tesla") #=> {:error, {:unexpected_response, "not ok :("}}

[yordis]

@fuelen I am updating the documentation, it works as intended, but you must place the telemetry as close as possible to the actual request; the issue is that your ExpectOKMiddleware is transforming the response into an error tuple; so you los the information; here is working test case:

defmodule TelemetryTest do
  use ExUnit.Case

  defmodule ExpectOKMiddleware do
    @behaviour Tesla.Middleware

    @impl Tesla.Middleware
    def call(env, next, _opts) do
      with {:ok, env} <- Tesla.run(env, next) do
        if dbg(env).status == 200 do
          {:ok, env}
        else
          {:error, {:unexpected_response, "not ok :("}}
        end
      end
    end
  end

  test "assering proper telemetry metadata" do
    :telemetry.attach_many(
      "tesla.request",
      [[:tesla, :request, :stop]],
      fn _event, measurements, metadata, _ ->
        send(self(), {:telemetry, metadata})
      end,
      nil
    )

    client =
      Tesla.client([
        {Tesla.Middleware.BaseUrl, "https://hex.pm/api"},
        ExpectOKMiddleware,
        Tesla.Middleware.Telemetry,
      ])

    Tesla.get(client, "/packages/tesla")

    assert_receive {:telemetry, metadata}
    assert metadata.env.status == 400
  end
end