Log in Prometheus current number of host and index

This is done in 3 places which should not be called in high frequency

ee/changelogs/unreleased/load-balancing-prometheus.yml

+---
+title: 'DB Load Balancing: Log Prometheus current number of hosts and current index'
+merge_request: 15440
+author:
+type: changed

ee/lib/gitlab/database/load_balancing/host_list.rb

@@ -10,6 +10,10 @@ module Gitlab
           @hosts = hosts.shuffle
           @index = 0
           @mutex = Mutex.new
+          @hosts_gauge = Gitlab::Metrics.gauge(:db_load_balancing_hosts, 'Current number of load balancing hosts')
+          @index_gauge = Gitlab::Metrics.gauge(:db_load_balancing_index, 'Current load balancing host index')
+
+          set_metrics!
         end
 
         def hosts
@@ -29,13 +33,24 @@ module Gitlab
             @hosts = hosts.shuffle
             @index = 0
           end
+
+          set_metrics!
+        end
+
+        # Sets metrics before returning next host
+        def next
+          next_host.tap do |_|
+            set_metrics!
+          end
         end
 
+        private
+
         # Returns the next available host.
         #
         # Returns a Gitlab::Database::LoadBalancing::Host instance, or nil if no
         # hosts were available.
-        def next
+        def next_host
           @mutex.synchronize do
             break if @hosts.empty?
 
@@ -53,6 +68,11 @@ module Gitlab
             end
           end
         end
+
+        def set_metrics!
+          @hosts_gauge.set({}, @hosts.length)
+          @index_gauge.set({}, @index)
+        end
       end
     end
   end

ee/spec/lib/gitlab/database/load_balancing/host_list_spec.rb

 require 'spec_helper'
 
 describe Gitlab::Database::LoadBalancing::HostList do
+  def expect_metrics(hosts, index)
+    expect(Gitlab::Metrics.registry.get(:db_load_balancing_hosts).get({})).to eq(hosts)
+    expect(Gitlab::Metrics.registry.get(:db_load_balancing_index).get({})).to eq(index)
+  end
+
   before do
     allow(Gitlab::Database)
       .to receive(:create_connection_pool)
@@ -17,6 +22,14 @@ describe Gitlab::Database::LoadBalancing::HostList do
     described_class.new(hosts)
   end
 
+  describe '#initialize' do
+    it 'sets metrics for current number of hosts and current index' do
+      host_list
+
+      expect_metrics(2, 0)
+    end
+  end
+
   describe '#length' do
     it 'returns the number of hosts in the list' do
       expect(host_list.length).to eq(2)
@@ -63,6 +76,7 @@ describe Gitlab::Database::LoadBalancing::HostList do
 
       expect(host_list.length).to eq(1)
       expect(host_list.hosts[0].host).to eq('foo')
+      expect_metrics(1, 0)
     end
   end
 
@@ -74,14 +88,20 @@ describe Gitlab::Database::LoadBalancing::HostList do
 
     it 'cycles through all available hosts' do
       expect(host_list.next).to eq(host_list.hosts[0])
+      expect_metrics(2, 1)
+
       expect(host_list.next).to eq(host_list.hosts[1])
+      expect_metrics(2, 0)
+
       expect(host_list.next).to eq(host_list.hosts[0])
+      expect_metrics(2, 1)
     end
 
     it 'skips hosts that are offline' do
       allow(host_list.hosts[0]).to receive(:online?).and_return(false)
 
       expect(host_list.next).to eq(host_list.hosts[1])
+      expect_metrics(2, 0)
     end
 
     it 'returns nil if no hosts are online' do
@@ -90,6 +110,7 @@ describe Gitlab::Database::LoadBalancing::HostList do
       end
 
       expect(host_list.next).to be_nil
+      expect_metrics(2, 0)
     end
 
     it 'returns nil if no hosts are available' do