Cast values to strings before escaping them

This ensures that e.g. line numbers used in tags are first casted to
strings.

app/workers/metrics_worker.rb

@@ -24,6 +24,6 @@ class MetricsWorker
   end
 
   def escape_value(value)
-    value.gsub('=', '\\=')
+    value.to_s.gsub('=', '\\=')
   end
 end

spec/workers/metrics_worker_spec.rb

@@ -36,5 +36,9 @@ describe MetricsWorker do
     it 'escapes an equals sign' do
       expect(worker.escape_value('foo=')).to eq('foo\\=')
     end
+
+    it 'casts values to Strings' do
+      expect(worker.escape_value(10)).to eq('10')
+    end
   end
 end