A working, but ugly way to fetch from a url

chef-http-client.rb

ruby_block 'get build' do
  block do
    require "net/https"
    require "uri"
    require "json"

    uri = URI.parse("http://nexus.example.com/nexus/service/local/repositories/foobar/content/master-5678.zip?describe=info")
    http = Net::HTTP.new(uri.host, uri.port)
    request = Net::HTTP::Get.new(uri.request_uri)
    request.initialize_http_header({ "Accept" => "application/json" })

    response = http.request(request)
    metadata = JSON.parse(response.body)
    # metadata['data']['md5Hash'] is a string like "1234"
    # Save the string to the node_state https://docs.chef.io/recipes.html#node-run-state
    node.run_state['artifact_md5'] = metadata['data']['md5Hash']
  end
  action :run
end

The following works as a library

module Foo
  module Helper

    def nexus_md5
      require 'json'
      artifact_info = Chef::HTTP.new("http://nexus.example.com/nexus/service/local/repositories/foobar/content/master-5678.zip?describe=info")
      headers = { "Accept" => "application/json" }
      response_xml = artifact_info.request('get',artifact_info.url)
      response_json = JSON.parse( artifact_info.request('get', artifact_info.url, headers ) )
      return response_json['data']['md5Hash']
    end
  end
end

However it has a huge limitation in that remote_file only works with sha256 hashes. Your artifact repository would need to expose that metadata aswell.