Drop db_load_balancing_index gauge metric

As the host list is shuffled per rails process, this metric is
meaningless and should not be relied upon. So we are removing this
metric.

doc/administration/monitoring/prometheus/gitlab_metrics.md

@@ -140,8 +140,7 @@ The following metrics are available:
 
 | Metric                            | Type      | Since                                                         | Description                            |
 |:--------------------------------- |:--------- |:------------------------------------------------------------- |:-------------------------------------- |
-| `db_load_balancing_hosts`         | Gauge     | [12.3](https://gitlab.com/gitlab-org/gitlab/issues/13630)  | Current number of load balancing hosts |
-| `db_load_balancing_index`         | Gauge     | [12.3](https://gitlab.com/gitlab-org/gitlab/issues/13630)  | Current load balancing host index      |
+| `db_load_balancing_hosts`         | Gauge     | [12.3](https://gitlab.com/gitlab-org/gitlab/issues/13630)     | Current number of load balancing hosts |
 
 ## Ruby metrics
 

ee/changelogs/unreleased/drop-db_load_balancing_index.yml

+---
+title: Remove db_load_balancing_index gauge metric
+merge_request: 17561
+author:
+type: removed

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

@@ -11,7 +11,6 @@ module Gitlab
           @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
@@ -71,7 +70,6 @@ module Gitlab
 
         def set_metrics!
           @hosts_gauge.set({}, @hosts.length)
-          @index_gauge.set({}, @index)
         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)
+  def expect_metrics(hosts)
     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
@@ -26,7 +25,7 @@ describe Gitlab::Database::LoadBalancing::HostList do
     it 'sets metrics for current number of hosts and current index' do
       host_list
 
-      expect_metrics(2, 0)
+      expect_metrics(2)
     end
   end
 
@@ -76,7 +75,7 @@ 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)
+      expect_metrics(1)
     end
   end
 
@@ -88,20 +87,20 @@ 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_metrics(2)
 
       expect(host_list.next).to eq(host_list.hosts[1])
-      expect_metrics(2, 0)
+      expect_metrics(2)
 
       expect(host_list.next).to eq(host_list.hosts[0])
-      expect_metrics(2, 1)
+      expect_metrics(2)
     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)
+      expect_metrics(2)
     end
 
     it 'returns nil if no hosts are online' do
@@ -110,7 +109,7 @@ describe Gitlab::Database::LoadBalancing::HostList do
       end
 
       expect(host_list.next).to be_nil
-      expect_metrics(2, 0)
+      expect_metrics(2)
     end
 
     it 'returns nil if no hosts are available' do