Drop empty tag values from metrics

InfluxDB throws an error when trying to store a list of tags where one
or more have an empty value.

app/workers/metrics_worker.rb

@@ -16,8 +16,12 @@ class MetricsWorker
       new_hash = hash.symbolize_keys
 
       new_hash[:tags].each do |key, value|
+        if value.blank?
+          new_hash[:tags].delete(key)
+        else
           new_hash[:tags][key] = escape_value(value)
         end
+      end
 
       new_hash
     end

spec/workers/metrics_worker_spec.rb

@@ -30,6 +30,14 @@ describe MetricsWorker do
 
       expect(metrics).to eq([{ values: {}, tags: { 'foo' => 'bar\\=' } }])
     end
+
+    it 'drops empty tags' do
+      metrics = worker.prepare_metrics([
+        { 'values' => {}, 'tags' => { 'cats' => '', 'dogs' => nil }}
+      ])
+
+      expect(metrics).to eq([{ values: {}, tags: {} }])
+    end
   end
 
   describe '#escape_value' do