Skip recording ES-metrics when there are none

This skips calling out into the Prometheus Client library entirely for
requests that didn't call out to ElasticSearch.

Incrementing the counter by 0 would be a no-op anyway.

lib/gitlab/metrics/elasticsearch_rack_middleware.rb

@@ -24,12 +24,12 @@ module Gitlab
         query_time = ::Gitlab::Instrumentation::ElasticsearchTransport.query_time
         request_count = ::Gitlab::Instrumentation::ElasticsearchTransport.get_request_count
 
+        return unless request_count > 0
+
         transaction.increment(:http_elasticsearch_requests_total, request_count) do
           docstring 'Amount of calls to Elasticsearch servers during web requests'
         end
 
-        return unless request_count > 0
-
         transaction.observe(:http_elasticsearch_requests_duration_seconds, query_time) do
           docstring 'Query time for Elasticsearch servers during web requests'
           buckets HISTOGRAM_BUCKETS

spec/lib/gitlab/metrics/elasticsearch_rack_middleware_spec.rb

@@ -42,8 +42,9 @@ RSpec.describe Gitlab::Metrics::ElasticsearchRackMiddleware do
     context 'when there are no elasticsearch requests' do
       let(:elasticsearch_requests_count) { 0 }
 
-      it 'does not record duration if there were no ES calls' do
+      it 'does not record any metrics' do
         expect(transaction).not_to receive(:observe).with(:http_elasticsearch_requests_duration_seconds)
+        expect(transaction).not_to receive(:increment).with(:http_elasticsearch_requests_total, 0)
 
         middleware.call(env)
       end