Add more Praefect to scrape config

Each Praefect votes on the promotion of Gitaly nodes to primary, and
there should not be a single point of failure. Three Praefects is
preferred to two to prevent a tie. The scrape config should also
reflect this to make it clear that the Praefect load balancer IP is
not the correct address for this configuration.

doc/administration/gitaly/praefect.md

@@ -46,7 +46,7 @@ A minimum highly available configuration requires:
 
 - 1 highly available load balancer
 - 1 highly available PostgreSQL server (PostgreSQL 9.6 or newer)
-- 2 Praefect nodes
+- 3 Praefect nodes
 - 3 Gitaly nodes (1 primary, 2 secondary)
 
 ![Architecture diagram](img/praefect_architecture_v12_10.png)
@@ -613,7 +613,9 @@ Particular attention should be shown to:
        'job_name' => 'praefect',
        'static_configs' => [
          'targets' => [
-           'PRAEFECT_HOST:9652' # praefect
+           'PRAEFECT_HOST:9652', # praefect-1
+           'PRAEFECT_HOST:9652', # praefect-2
+           'PRAEFECT_HOST:9652', # praefect-3
          ]
        ]
      },