Track new Redis connections

Increment the counter `new_redis_connections` on each call to
`Redis::Client#connect`, if we're in a transaction.

CHANGELOG

@@ -22,6 +22,7 @@ v 8.9.0 (unreleased)
   - Reduce number of fog gem dependencies
   - Remove project notification settings associated with deleted projects
   - Fix 404 page when viewing TODOs that contain milestones or labels in different projects
+  - Add a metric for the number of new Redis connections created by a transaction
   - Redesign navigation for project pages
   - Fix groups API to list only user's accessible projects
   - Redesign account and email confirmation emails

config/initializers/metrics.rb

@@ -138,4 +138,20 @@ if Gitlab::Metrics.enabled?
   GC::Profiler.enable
 
   Gitlab::Metrics::Sampler.new.start
+
+  module TrackNewRedisConnections
+    def connect(*args)
+      val = super
+
+      if current_transaction = Gitlab::Metrics::Transaction.current
+        current_transaction.increment(:new_redis_connections, 1)
+      end
+
+      val
+    end
+  end
+
+  class ::Redis::Client
+    prepend TrackNewRedisConnections
+  end
 end