Add support for Req http client (draft)

[krns]

Hello, I would be happy if Req would be supported as http client lib. Here is a working proof of concept.

Since Hackney is the only HTTP client that is supported so far, I had to bend a little in some places. A few comments on this below.

One general question:

Because Req is not known in the Sentry library I get a few warning like:

warning: Req.new/1 is undefined (module Req is not available or is yet to be defined). Make sure the module name is correct and has been specified in full (or that an alias has been defined)
warning: Req.merge/2 is undefined (module Req is not available or is yet to be defined). Make sure the module name is correct and has been specified in full (or that an alias has been defined)
warning: Req.post/1 is undefined (module Req is not available or is yet to be defined). Make sure the module name is correct and has been specified in full (or that an alias has been defined)

Should I make use of Code.ensure_loaded?(Req) and Application.ensure_all_started(:req)?

[krns]

I don't like this Req-specific change inside the test command but I left it like this for now because I wanted to get some general feedback first.

Details why this is needed: https://elixirforum.com/t/argumenterror-unknown-registry-req-finch-on-a-simple-http-request-using-req-library/64638/9

[krns]

We should pattern match when Req is available.

Suggested change

	{:ok, %{status: status, headers: headers, body: body}} = result -> {:ok, status, headers, body}
	{:ok, %Req.Response{status: status, headers: headers, body: body}} = result -> {:ok, status, headers, body}

I transform the result into the Hackney-inspired form so everything else works.

[krns]

Req decodes responses automatically. I choose to disable body decoding so the users can make use of the json_library option.

[krns]

I removed the is_list guard because it's highly Hackney-specific (Req response headers are a Map).

[whatyouhide]

@krns thanks for the PR, but I don't think this is the right approach. This SDK already supports the Sentry.HTTPClient behaviour. With that, you can use any custom HTTP client (which includes Req) provided you will write a translation layer for it. For example, here's how to use Finch—a blog post from 4 years ago.

We are already looking at replacing the default Hackney-based client with one based on Finch (#758), but I’m not convinced this is worth it if Erlang/Elixir will ever get a stdlib HTTP client. We might opt for Mint instead if they don't, since it will be the lowest-level and hopefully most-widely-adopted-as-base client.

Hopefully this all makes sense. Cheers!