Add support for empty response bodies

[JoelQ]

🧰 What's being changed?

Some responses have no response body (e.g. an HTTP 204 response).
Rack::Response handles this by returning nil when calling #body.

The HAR spec mentions that response.content.text key is optional.
However, it does require the response.content.mimeType key. Based on
discussion with the Readme.io team, we decided to send an empty string in
this spot when there is no request body.

The HAR spec is ambiguous about the response.content.size key. It
mentions that it should be 0 for 304 responses (another scenario with no
body) and -1 if unknown. I chose to set 0 for responses without a body
since we do know the length and the spec specifically says to use 0
in some instances of no-body responses.

While testing this code, I found a bug where attempting to read the body
more than once failed since the IO object had not been rewound.

While moving some of the existing JSON code around, I realized that some
of it's code paths were untested so I added a test around invalid
payloads.

🧪 Testing

Create a simple Rack app that returns an empty body:

# config.ru

$LOAD_PATH << File.expand_path("../path/to/metrics-sdks/packages/ruby/lib", __FILE__)
require "readme/metrics"

class ApiApp
  def call(env)
    [204, {}, []]
  end
end

map "/api" do
  use Readme::Metrics, buffer_length: 1, api_key: "YOUR_API_KEY" do
    { id: 1, label: "Joël", email: "[email protected]" }
  end
  run ApiApp.new
end

run it with:

rackup

and make a request to it:

curl 'http://localhost:9292/api/foo' -X POST

Then expect to see the request show up in the Readme.io dashboard

[erunion]

You might want to add support for JSON variants like application/x-json, text/json, or vendored JSON content types like application/vnd.github.v3.star+json.

I added something similar last week in one of our core OpenAPI tooling methods last week here: https://github.com/readmeio/oas/blob/master/packages/tooling/src/operation.js#L62

[JoelQ]

Good call. Since this PR is focused on empty response bodies and only moved the JSON code, I'm going to implement your suggestion in a separate feature. I've created a Trello card to keep track of it.

[thestrabusiness]

Good call 👍