puppet: Add postgresql-client depenencies to monitoring.

The `unless` step errors out if /usr/bin/psql does not exist at
first evaluation time -- protect that with a `test -f` check, and
protect the actual `createuser` with a dependency on `postgresql-client`.
To work around `Zulip::Safepackage` not actually being safe to
instantiate more than once, we move the instantiation of
`Package[postgresql-client]` into a class which can be safely
included one or more times.

puppet/zulip/manifests/app_frontend_base.pp

@@ -10,10 +10,8 @@ class zulip::app_frontend_base {
   if $::os['family'] == 'Debian' {
     # Upgrade and other tooling wants to be able to get a database
     # shell.  This is not necessary on CentOS because the PostgreSQL
-    # package already includes the client.  This may get us a more
-    # recent client than the database server is configured to be,
-    # ($zulip::postgresql_common::version), but they're compatible.
-    zulip::safepackage { 'postgresql-client': ensure => installed }
+    # package already includes the client.
+    include zulip::postgresql_client
   }
   # For Slack import
   zulip::safepackage { 'unzip': ensure => installed }

puppet/zulip/manifests/postgresql_client.pp

@@ -0,0 +1,8 @@
+class zulip::postgresql_client {
+  # This may get us a more recent client than the database server is
+  # configured to be, ($zulip::postgresql_common::version), but
+  # they're compatible.
+  package { 'postgresql-client':
+    ensure => installed,
+  }
+}

puppet/zulip_ops/manifests/prometheus/postgresql.pp

@@ -47,9 +47,11 @@ class zulip_ops::prometheus::postgresql {
     require => Exec['compile postgres_exporter'],
   }
 
+  include zulip::postgresql_client
   exec { 'create prometheus postgres user':
+    require => Package['postgresql-client'],
     command => '/usr/bin/createuser -g pg_monitor prometheus',
-    unless  => '/usr/bin/psql -tAc "select usename from pg_user" | /bin/grep -xq prometheus',
+    unless  => 'test -f /usr/bin/psql && /usr/bin/psql -tAc "select usename from pg_user" | /bin/grep -xq prometheus',
     user    => 'postgres',
   }