Makes spaces optional in observability plugin (#151147)

## Summary

This PR makes spaces optional in the "observability" plugin as requested
in #149687

To test this, use the following setting in your kibana.yml config file:

```yaml
xpack.spaces.enabled: false
```

When you log in, there will be no spaces choice. 

<img width="2543" alt="Screenshot 2023-02-21 at 11 53 18 AM"
src="https://user-images.githubusercontent.com/159370/220409537-43a216d5-81c9-4b29-97d8-47705bdacd06.png">

x-pack/plugins/observability/kibana.jsonc

@@ -24,8 +24,7 @@
       "unifiedSearch",
       "security",
       "guidedOnboarding",
-      "share",
-      "spaces"
+      "share"
     ],
     "optionalPlugins": [
       "discover",

x-pack/plugins/observability/public/hooks/use_kibana_space.tsx

@@ -15,7 +15,7 @@ export const useKibanaSpace = () => {
     data: space,
     loading,
     error,
-  } = useFetcher<Promise<Space>>(() => {
+  } = useFetcher<Promise<Space> | undefined>(() => {
     return services.spaces?.getActiveSpace();
   }, [services.spaces]);
 

x-pack/plugins/observability/public/pages/overview/containers/overview_page/helpers/use_metrics.ts

@@ -29,9 +29,22 @@ export const useOverviewMetrics = ({ hasAnyData }: { hasAnyData: boolean | undef
     }
 
     CAPABILITIES_KEYS.forEach((feature) => {
-      if (capabilities[feature].show === false) {
+      const name = feature === 'infrastructure' ? 'metrics' : feature;
+
+      // Track metric if the feature has been disabled, either because it
+      // is missing or has show === false (manual disabling may not be
+      // possible in all versions of Kibana)
+      if (!capabilities[feature] || capabilities[feature]?.show === false) {
+        trackMetric({
+          metric: `oblt_disabled_feature_${name}`,
+        });
+      }
+
+      // Track a separate metric if the feature is missing from the capabilities
+      // (This usually means the plugin was auto-disabled by Kibana)
+      if (!capabilities[feature]) {
         trackMetric({
-          metric: `oblt_disabled_feature_${feature === 'infrastructure' ? 'metrics' : feature}`,
+          metric: `oblt_missing_feature_${name}`,
         });
       }
     });

x-pack/plugins/observability/public/plugin.ts

@@ -105,7 +105,7 @@ export interface ObservabilityPublicPluginsStart {
   ruleTypeRegistry: RuleTypeRegistryContract;
   security: SecurityPluginStart;
   share: SharePluginStart;
-  spaces: SpacesPluginStart;
+  spaces?: SpacesPluginStart;
   triggersActionsUi: TriggersAndActionsUIPublicPluginStart;
   usageCollection: UsageCollectionSetup;
   unifiedSearch: UnifiedSearchPublicPluginStart;

x-pack/plugins/observability/server/plugin.ts

@@ -17,6 +17,7 @@ import { PluginSetupContract } from '@kbn/alerting-plugin/server';
 import { Dataset, RuleRegistryPluginSetupContract } from '@kbn/rule-registry-plugin/server';
 import { PluginSetupContract as FeaturesSetup } from '@kbn/features-plugin/server';
 import { createUICapabilities } from '@kbn/cases-plugin/common';
+import { SpacesPluginSetup } from '@kbn/spaces-plugin/server';
 import { experimentalRuleFieldMap } from '@kbn/rule-registry-plugin/common/assets/field_maps/experimental_rule_field_map';
 import { ECS_COMPONENT_TEMPLATE_NAME } from '@kbn/alerting-plugin/server';
 import type { GuidedOnboardingPluginSetup } from '@kbn/guided-onboarding-plugin/server';
@@ -50,6 +51,7 @@ interface PluginSetup {
   features: FeaturesSetup;
   guidedOnboarding: GuidedOnboardingPluginSetup;
   ruleRegistry: RuleRegistryPluginSetupContract;
+  spaces?: SpacesPluginSetup;
   usageCollection?: UsageCollectionSetup;
 }