feat: add new event metadata to tesla telemetry

[polvalente]

This PR aims to provide a way for relaying through the tesla.request.stop event the unresolved path when using both Telemetry and PathParams middlewares.

The issue with the current implementation is that in success cases, it's impossible to retrieve the unresolved path from the event metadata

[teamon]

I see. Assuming you use Telemetry middleware before PathParams I think the best way would be to use something similar to KeepRequest:

defmodule KeepRequestURL do
  def call(env, next, _opts) do
    env
    |> Tesla.put_opt(:req_url, env.url)
    |> Tesla.run(next)
  end
end

# then in client
plug Tesla.Middleware.Telemetry
plug Tesla.Middleware.KeepRequestURL
plug Tesla.Middleware.PathParams

Then in you telemetry handlers you should be able to access the original URL via env.opts[:req_url].

I'd be happy to accept a PR adding :req_url to the built-in KeepRequest middleware too.

[polvalente]

I see. Assuming you use Telemetry middleware before PathParams I think the best way would be to use something similar to KeepRequest:

defmodule KeepRequestURL do
  def call(env, next, _opts) do
    env
    |> Tesla.put_opt(:req_url, env.url)
    |> Tesla.run(next)
  end
end

# then in client
plug Tesla.Middleware.Telemetry
plug Tesla.Middleware.KeepRequestURL
plug Tesla.Middleware.PathParams

Then in you telemetry handlers you should be able to access the original URL via env.opts[:req_url].

I'd be happy to accept a PR adding :req_url to the built-in KeepRequest middleware too.

Yeah, the Telemetry middleware is before PathParams, but without this fix the URL ends up being resolved for happy-path scenarios, and thus this PR.

I'll change this PR to adding :req_url to the KeepRequest middleware then, a I'll also take the time to add an example to the Telemetry docs.

Thanks for the review and the suggestion!

[polvalente]

@teamon the PR has now been updated as per your suggestion. I took the liberty to add an "integration" test for KeepRequest + PathParams + Telemetry, as well as an example for other Tesla.Middleware.Telemetry users.

If there's a better place to add these, please let me know!

[teamon]

Nice! 😍