特定ホスト に対する HTTP リクエスト・レスポンスをコピーする proxy 類似 HTTPサーバ

特定ホストしか使えないので、http proxy ではない何か
Web API の、API Base を変えて遊ぶときに使えるかも (https の通信内容を見るよりは容易?)
以下の例では、mackerel.io の mackerel-agent で apibase = "http://localhost:4567" という設定をするとそのまま使える

require 'sinatra'
require 'net/http'
require 'uri'
require 'pathname'
require 'logger'
require 'pp'

mackerel = 'https://mackerel.io'
mackerel_url = URI.parse(mackerel)

logdir = Pathname.new(__FILE__).parent + 'log'
logdir.directory? || logdir.mkpath
logfile = logdir + Time.now.strftime('%Y-%m-%d')
logio = logfile.open('a')
logger = Logger.new(logio)

[:get, :post, :delete, :patch, :put, :head, :options].each do |method|
  send method, '*' do
    path_with_query = request.env['REQUEST_PATH']
    if request.env['rack.request.query_string'] && request.env['rack.request.query_string'] != ''
      path_with_query << '?' + request.env['rack.request.query_string']
    end

    server_req = Net::HTTP.const_get(request.env['REQUEST_METHOD'].downcase.capitalize).new(path_with_query)
    if body = env["rack.input"].gets
      server_req.body = body

      if request.env['CONTENT_TYPE'] = 'application/json'
        logger << [Time.now.strftime('%Y-%m-%d %H:%M:%S'), path_with_query, body].join("\t")
        logio.flush
      end
    end

    http_headers = request.env.select { |k, v| k.start_with?('HTTP_') }
    headers = http_headers.inject({}) do |a, (k, v)|
      a[k.sub(/^HTTP_/, "").downcase.gsub(/(^|_)\w/) { |word| word.upcase }.gsub("_", "-") ] = v
      a
    end
    headers.each do |key, val|
      server_req.add_field(key, val)
    end

    # special Header
    server_req['HOST'] = mackerel_url.host
    server_req['Content-Type'] = request.env['CONTENT_TYPE']

    http = Net::HTTP.new(mackerel_url.host, mackerel_url.port)
    if mackerel_url.scheme == 'https'
      http.use_ssl = true
    end

    server_resp = nil
    http.start do |h|
      server_resp = h.request(server_req)
    end

    server_resp.each do |key, val|
      response[key] = val
    end
    response.header.delete('content-length')

    status server_resp.code
    return server_resp.body
  end
end