Add packet flow metric query for network policy metrics

This query will be used to collect network policy packet stats usage
metrics from clusters.

ee/lib/gitlab/prometheus/queries/packet_flow_metrics_query.rb

+# frozen_string_literal: true
+
+module Gitlab::Prometheus::Queries
+  class PacketFlowMetricsQuery < BaseQuery
+    def query(namespace)
+      total_query = sum_by_verdict(
+        increase_query(%{destination="#{namespace}"}),
+        increase_query(%{source="#{namespace}"})
+      )
+
+      transform_sum_result(client_query(total_query))
+    end
+
+    private
+
+    def sum_by_verdict(vec1, vec2)
+      %{sum by(verdict) (#{vec1} or on(source,destination,verdict) #{vec2})}
+    end
+
+    def increase_query(selector)
+      %{increase(hubble_flows_processed_total{#{selector}}[1w])}
+    end
+
+    # Returns the number of forwarded and dropped packets from an instant vector:
+    # [
+    #  {"metric"=>{"verdict"=>"FORWARDED"}, "value"=>[1582231596.64, "73772.43143284984"]},
+    #  {"metric"=>{"verdict"=>"DROPPED"}, "value"=>[1582231596.64, "5.002730665588791"]}
+    # ]
+    def transform_sum_result(vector)
+      values = vector.each_with_object({ "DROPPED" => 0, "FORWARDED" => 0 }) do |val, acc|
+        acc[val["metric"].first.second] = val["value"].last.to_i
+      end
+
+      { forwards: values["FORWARDED"], drops: values["DROPPED"] }
+    end
+  end
+end

ee/spec/lib/gitlab/prometheus/queries/packet_flow_metrics_query_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe Gitlab::Prometheus::Queries::PacketFlowMetricsQuery do
+  let(:namespace) { 'query-12345678-production' }
+  let(:query_response) do
+    [
+      { "metric" => { "verdict" => "FORWARDED" }, "value" => [1582231596.64, "73772.43143284984"] },
+      { "metric" => { "verdict" => "DROPPED" }, "value" => [1582231596.64, "5.002730665588791"] }
+    ]
+  end
+  let(:client) { double('prometheus_client', query: query_response) }
+
+  subject { described_class.new(client) }
+
+  describe '#query' do
+    it 'sends prometheus query' do
+      query = 'sum by(verdict) (' \
+              'increase(hubble_flows_processed_total{destination="query-12345678-production"}[1w])' \
+              ' or on(source,destination,verdict) ' \
+              'increase(hubble_flows_processed_total{source="query-12345678-production"}[1w]))'
+      subject.query(namespace)
+      expect(client).to have_received(:query).with(query)
+    end
+
+    it 'returns metrics' do
+      result = subject.query(namespace)
+      expect(result).to match(forwards: 73772, drops: 5)
+    end
+  end
+end