WEBrick::HTTPServlet::AbstractServlet with CORS enabled (works on Vercel with Ruby runtime)

rack.rb

require 'rack'
require 'webrick'
require 'json'

Response = Struct.new(:status, :headers, :body)
app = Proc.new do |handler|
  handler = Response.new
  handler.status = 200   
  handler.headers = {
    'Content-Type' => 'application/json'
  }
  handler.body = [
    {'Message': 'Hello, world!'}.to_json
  ]
  [handler.status, handler.headers, handler.body]
end

Rack::Handler::WEBrick.run app

serverless.rb

require 'net/http'

Handler = Proc.new do |req, res|
  coord = {
    lat,
    lon,
    key
  }
  
  host = "https://api.openweathermap.org/data/2.5/onecall?lat=#{coord[:lat]}&lon=#{coord[:lon]}&appid=#{coord[:key]}"
  uri = URI(host)
  response = Net::HTTP.get(uri)

  res.status = 200
  res['Content-Type'] = 'text/plain'
  res.header['Access-Control-Allow-Origin'] = 'https://endpoint.domain'
  res.header['Access-Control-Allow-Methods'] = 'GET'
  res.header['Vary'] = 'Access-Control-Request-Headers'
  res.header['Access-Control-Allow-Headers'] = 'Content-Type, Accept'
  res.header['Access-Control-Request-Method'] = '*'
  res['Access-Control-Allow-Credentials'] = true
  res.body = response
end