Make it possible to set instrumenter and wrap http adapter

Instrumentation is important in production (for metrics) and dev (for
logging).

Wrapping the adapter is important because it allows for caching,
instrumentation and more.

Also added docs for the cloud method.

lib/flipper/cloud.rb

@@ -3,17 +3,39 @@
 
 module Flipper
   module Cloud
-    def self.new(token, options = {})
-      url = options.fetch(:url, "https://www.featureflipper.com/adapter")
-      instrumenter = options.fetch(:instrumenter, Flipper::Instrumenters::Noop)
+    # The default adapter wrapper which doesn't wrap at all.
+    DEFAULT_ADAPTER_WRAPPER_BLOCK = ->(adapter) { adapter }
 
+    # The default url should be the one, the only, the website™.
+    DEFAULT_URL = "https://www.featureflipper.com/adapter"
+
+    # Public: Returns a new Flipper instance with an http adapter correctly
+    # configured for flipper cloud.
+    #
+    # token - The String token for the environment from the website.
+    # options - The Hash of options.
+    #           # :url - The url to point at (defaults to DEFAULT_URL).
+    #           # :adapter_wrapper - The adapter wrapper block. Block should
+    #                                receive an adapter and return an adapter.
+    #                                Allows you to wrap the http adapter with
+    #                                other adapters to make instrumentation and
+    #                                caching easy.
+    #           # :instrumenter - The optional instrumenter to use for the
+    #                             Flipper::DSL instance (defaults to Noop).
+    def self.new(token, options = {})
+      url = options.fetch(:url, DEFAULT_URL)
       http_options = {
         uri: URI(url),
         headers: {
           "Feature-Flipper-Token" => token,
         },
       }
       adapter = Flipper::Adapters::Http.new(http_options)
+
+      adapter_wrapper = options.fetch(:adapter_wrapper, DEFAULT_ADAPTER_WRAPPER_BLOCK)
+      adapter = adapter_wrapper.call(adapter)
+
+      instrumenter = options.fetch(:instrumenter, Flipper::Instrumenters::Noop)
       Flipper.new(adapter, instrumenter: instrumenter)
     end
   end

spec/flipper/cloud_spec.rb

@@ -1,5 +1,6 @@
 require 'helper'
 require 'flipper/cloud'
+require 'flipper/adapters/instrumented'
 
 RSpec.describe Flipper::Cloud do
   context "initialize with token" do
@@ -52,4 +53,19 @@
       expect(uri.path).to eq('/sadpanda')
     end
   end
+
+  it 'can set instrumenter' do
+    instrumenter = Object.new
+    instance = described_class.new('asdf', instrumenter: instrumenter)
+    expect(instance.instrumenter).to be(instrumenter)
+  end
+
+  it 'allows wrapping adapter with another adapter like the instrumenter' do
+    adapter_wrapper = ->(adapter) {
+      Flipper::Adapters::Instrumented.new(adapter)
+    }
+    instance = described_class.new('asdf', adapter_wrapper: adapter_wrapper)
+    # instance.adapter is memoizable adapter instance
+    expect(instance.adapter.adapter).to be_instance_of(Flipper::Adapters::Instrumented)
+  end
 end