1 files changed, 40 insertions, 0 deletions
diff --git a/lib/gitlab/instrumentation/connection_pool.rb b/lib/gitlab/instrumentation/connection_pool.rb
new file mode 100644
index 00000000000..76e6af34054
--- /dev/null
+++ b/lib/gitlab/instrumentation/connection_pool.rb
@@ -0,0 +1,40 @@
+# frozen_string_literal: true
+
+module Gitlab
+  module Instrumentation
+    # rubocop:disable Gitlab/ModuleWithInstanceVariables -- this module patches ConnectionPool to instrument it
+    module ConnectionPool
+      def initialize(options = {}, &block)
+        @name = options.fetch(:name, 'unknown')
+
+        super
+      end
+
+      def checkout(options = {})
+        conn = super
+
+        connection_class = conn.class.to_s
+        track_available_connections(connection_class)
+        track_pool_size(connection_class)
+
+        conn
+      end
+
+      def track_pool_size(connection_class)
+        # this means that the size metric for this pool key has been sent
+        return if @size_gauge
+
+        @size_gauge ||= ::Gitlab::Metrics.gauge(:gitlab_connection_pool_size, 'Size of connection pool', {}, :all)
+        @size_gauge.set({ pool_name: @name, pool_key: @key, connection_class: connection_class }, @size)
+      end
+
+      def track_available_connections(connection_class)
+        @available_gauge ||= ::Gitlab::Metrics.gauge(:gitlab_connection_pool_available_count,
+          'Number of available connections in the pool', {}, :all)
+
+        @available_gauge.set({ pool_name: @name, pool_key: @key, connection_class: connection_class }, available)
+      end
+    end
+    # rubocop:enable Gitlab/ModuleWithInstanceVariables
+  end
+end