Loki microservices Tanka: fix error during manifest generation (#9362)

**What this PR does / why we need it**:
Fixes an error on the ServiceMonitor generated when
```
memberlist_ring_enabled: true,
create_service_monitor: true,
```
It also adds some checks on `memberlist.libsonnet` to avoid generating
invalid kubernetes manifests.

**Which issue(s) this PR fixes**:
Fixes #9361

production/ksonnet/loki/memberlist.libsonnet

@@ -121,7 +121,7 @@
 
   compactor_statefulset+: if !$._config.memberlist_ring_enabled then {} else gossipLabel,
   distributor_deployment+: if !$._config.memberlist_ring_enabled then {} else gossipLabel,
-  index_gateway_statefulset+: if !$._config.memberlist_ring_enabled then {} else gossipLabel,
+  index_gateway_statefulset+: if !$._config.memberlist_ring_enabled || !$._config.use_index_gateway then {} else gossipLabel,
   ingester_statefulset: if $._config.multi_zone_ingester_enabled && !$._config.multi_zone_ingester_migration_enabled then {} else
     (super.ingester_statefulset + if !$._config.memberlist_ring_enabled then {} else gossipLabel),
   ingester_zone_a_statefulset+:
@@ -136,7 +136,7 @@
     if $._config.multi_zone_ingester_enabled && $._config.memberlist_ring_enabled
     then gossipLabel
     else {},
-  query_scheduler_deployment+: if !$._config.memberlist_ring_enabled then {} else gossipLabel,
+  query_scheduler_deployment+: if !$._config.memberlist_ring_enabled || !$._config.query_scheduler_enabled then {} else gossipLabel,
   ruler_deployment+: if !$._config.memberlist_ring_enabled || !$._config.ruler_enabled || $._config.stateful_rulers then {} else gossipLabel,
   ruler_statefulset+: if !$._config.memberlist_ring_enabled || !$._config.ruler_enabled || !$._config.stateful_rulers then {} else gossipLabel,
   // Headless service (= no assigned IP, DNS returns all targets instead) pointing to gossip network members.

production/ksonnet/loki/servicemonitor.libsonnet

@@ -41,7 +41,7 @@
           values: [
             this[name].spec.selector.name
             for name in std.objectFields(this)
-            if std.isObject(this[name]) && std.objectHas(this[name], 'kind') && this[name].kind == 'Service'
+            if std.isObject(this[name]) && std.objectHas(this[name], 'kind') && this[name].kind == 'Service' && std.objectHasAll(this[name].spec.selector, 'name')
           ],
         }],
       },