Add histogram and track latency

Added spec to ensure the latency was recorded.

app/services/spam/spam_verdict_service.rb

@@ -21,6 +21,8 @@ module Spam
         spamcheck_result, spamcheck_attribs = spamcheck_verdict
       end
 
+      histogram.observe( {}, external_spam_check_round_trip_time )
+
       # assign result to a var and log it before reassigning to nil when monitorMode is true
       original_spamcheck_result = spamcheck_result
 
@@ -95,5 +97,9 @@ module Spam
     def logger
       @logger ||= Gitlab::AppJsonLogger.build
     end
+
+    def histogram
+      @histogram ||= Gitlab::Metrics.histogram(:spamcheck_latency_seconds, 'Latency to the Spamcheck service')
+    end
   end
 end

spec/services/spam/spam_verdict_service_spec.rb

@@ -114,6 +114,19 @@ RSpec.describe Spam::SpamVerdictService do
         end
       end
     end
+
+    context 'records metrics' do
+      let(:histogram) { double('histogram') }
+
+      before do
+        expect(Gitlab::Metrics).to receive(:histogram).with(:spamcheck_latency_seconds, anything).and_return(histogram)
+      end
+
+      it 'records latency' do
+        expect(histogram).to receive(:observe)
+        subject
+      end
+    end
   end
 
   describe '#akismet_verdict' do