(POOLER-160) Add Prometheus Stats Feeds

Add a new Prometheus class as an additional stats feed along with the
existing feeds.

Move the metrics initialisation code into its own class and sub-class
the individual metrics implementations under this.

lib/vmpooler/metrics.rb

@@ -0,0 +1,24 @@
+# frozen_string_literal: true
+
+module Vmpooler
+  class Metrics
+    # static class instantiate appropriate metrics object.
+    def self.init(params)
+      if params[:statsd]
+        metrics = Vmpooler::Statsd.new(params[:statsd])
+      elsif params[:graphite]
+        metrics = Vmpooler::Graphite.new(params[:graphite])
+      elsif params[:prometheus]
+        metrics = Vmpooler::Promstats.new(params[:prometheus])
+      else
+        metrics = Vmpooler::DummyStatsd.new
+      end
+      metrics
+    end
+  end
+end
+
+require 'vmpooler/metrics/statsd'
+require 'vmpooler/metrics/dummy_statsd'
+require 'vmpooler/metrics/graphite'
+require 'vmpooler/metrics/promstats'