(POOLER-160) Add Prometheus to pooler startup

This is a re-architect of the vmpooler initialisation code to:
1. Allow an API service for both manager and the api
2. Add the Prometheus endpoints to the web service.
   Needed to change the way the Rack Service is started as instantiating
   using ".New" leads to a failure to initialise the http Stats
   collection.
3. Selectively load the pooler api and/or Prometheus endpoints.
4. Rework API Spec tests for revised API loading. Needed to tidy up the
   initialisation and perform a reset! after each test to avoid "leaks"
   and dependencies between the tests.

lib/vmpooler/api.rb

@@ -2,51 +2,52 @@
 
 module Vmpooler
   class API < Sinatra::Base
-    def initialize
-      super
-    end
-
-    not_found do
-      content_type :json
-
-      result = {
-        ok: false
-      }
-
-      JSON.pretty_generate(result)
-    end
-
-    # Load dashboard components
-    begin
-      require 'dashboard'
-    rescue LoadError
-      require File.expand_path(File.join(File.dirname(__FILE__), 'dashboard'))
-    end
-
-    use Vmpooler::Dashboard
-
     # Load API components
-    %w[helpers dashboard reroute v1].each do |lib|
-      begin
-        require "api/#{lib}"
-      rescue LoadError
-        require File.expand_path(File.join(File.dirname(__FILE__), 'api', lib))
-      end
+    %w[helpers dashboard reroute v1 request_logger].each do |lib|
+      require "vmpooler/api/#{lib}"
     end
+    # Load dashboard components
+    require 'vmpooler/dashboard'
 
-    use Vmpooler::API::Dashboard
-    use Vmpooler::API::Reroute
-    use Vmpooler::API::V1
-
-    def configure(config, redis, metrics)
+    def self.execute(torun, config, redis, metrics)
       self.settings.set :config, config
-      self.settings.set :redis, redis
+      self.settings.set :redis, redis unless redis.nil?
       self.settings.set :metrics, metrics
       self.settings.set :checkoutlock, Mutex.new
-    end
 
-    def execute!
-      self.settings.run!
+      # Deflating in all situations
+      # https://www.schneems.com/2017/11/08/80-smaller-rails-footprint-with-rack-deflate/
+      use Rack::Deflater
+
+      # not_found clause placed here to fix rspec test issue.
+      not_found do
+        content_type :json
+
+        result = {
+          ok: false
+        }
+
+        JSON.pretty_generate(result)
+      end
+
+      if metrics.respond_to?(:setup_prometheus_metrics)
+        # Prometheus metrics are only setup if actually specified
+        # in the config file.
+        metrics.setup_prometheus_metrics
+
+        use Prometheus::Middleware::Collector, metrics_prefix: "#{metrics.metrics_prefix}_http"
+        use Prometheus::Middleware::Exporter, path: metrics.endpoint
+      end
+
+      if torun.include? 'api'
+        use Vmpooler::Dashboard
+        use Vmpooler::API::Dashboard
+        use Vmpooler::API::Reroute
+        use Vmpooler::API::V1
+      end
+
+      # Get thee started O WebServer
+      self.run!
     end
   end
 end

lib/vmpooler/metrics/promstats.rb

@@ -1,5 +1,7 @@
 # frozen_string_literal: true
 
+require 'prometheus/client'
+
 module Vmpooler
   class Promstats < Metrics
     attr_reader :prefix, :endpoint, :metrics_prefix