[/api/stats] Add documentation + small improvement (#79330)

Co-authored-by: Christiane (Tina) Heiligers <christiane.heiligers@elastic.co>
Co-authored-by: Kibana Machine <42973632+kibanamachine@users.noreply.github.com>
Co-authored-by: Ahmad Bamieh <ahmadbamieh@gmail.com>

src/plugins/usage_collection/README.md

@@ -325,3 +325,8 @@ By storing these metrics and their counts as key-value pairs, we can add more me
 to worry about exceeding the 1000-field soft limit in Elasticsearch.
 
 The only caveat is that it makes it harder to consume in Kibana when analysing each entry in the array separately. In the telemetry team we are working to find a solution to this.
+
+# Routes registered by this plugin
+
+- `/api/ui_metric/report`: Used by `ui_metrics` usage collector instances to report their usage data to the server
+- `/api/stats`: Get the metrics and usage ([details](./server/routes/stats/README.md))

src/plugins/usage_collection/server/routes/stats/README.md

@@ -0,0 +1,20 @@
+# `/api/stats`
+
+This API returns the metrics for the Kibana server and usage stats. It allows the [Metricbeat Kibana module](https://www.elastic.co/guide/en/beats/metricbeat/current/metricbeat-module-kibana.html) to collect the [stats metricset](https://www.elastic.co/guide/en/beats/metricbeat/current/metricbeat-metricset-kibana-stats.html).
+
+By default, it returns the simplest level of stats; consisting of the Kibana server's ops metrics, version, status, and basic config like the server name, host, port, and locale.
+
+However, the information detailed above can be extended, with the combination of the following 3 query parameters:
+
+| Query Parameter | Default value | Description |
+|:----------------|:-------------:|:------------|
+|`extended`|`false`|When `true`, it adds `clusterUuid` and `usage`. The latter contains the information reported by all the Usage Collectors registered in the Kibana server. It may throw `503 Stats not ready` if any of the collectors is not fully initialized yet.|
+|`legacy`|`false`|By default, when `extended=true`, the key names of the data in `usage` are transformed into API-friendlier `snake_case` format (i.e.: `clusterUuid` is transformed to `cluster_uuid`). When this parameter is `true`, the data is returned as-is.|
+|`exclude_usage`|`false`|When `true`, and `extended=true`, it will report `clusterUuid` but no `usage`.|
+
+## Known use cases
+
+Metricbeat Kibana' stats metricset ([code](https://github.com/elastic/beats/blob/master/metricbeat/module/kibana/stats/stats.go)) uses this API to collect the metrics (every 10s) and usage (only once every 24h), and then reports them to the Monitoring cluster. They call this API in 2 ways:
+
+1. Metrics-only collection (every 10 seconds): `GET /api/stats?extended=true&legacy=true&exclude_usage=true`
+2. Metrics+usage (every 24 hours): `GET /api/stats?extended=true&legacy=true&exclude_usage=false`

src/plugins/usage_collection/server/routes/stats/index.ts

@@ -0,0 +1,20 @@
+/*
+ * Licensed to Elasticsearch B.V. under one or more contributor
+ * license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright
+ * ownership. Elasticsearch B.V. licenses this file to you under
+ * the Apache License, Version 2.0 (the "License"); you may
+ * not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+export { registerStatsRoute } from './stats';

src/plugins/usage_collection/server/routes/stats/stats.ts

@@ -30,8 +30,8 @@ import {
   MetricsServiceSetup,
   ServiceStatus,
   ServiceStatusLevels,
-} from '../../../../core/server';
-import { CollectorSet } from '../collector';
+} from '../../../../../core/server';
+import { CollectorSet } from '../../collector';
 
 const STATS_NOT_READY_MESSAGE = i18n.translate('usageCollection.stats.notReadyMessage', {
   defaultMessage: 'Stats are not ready yet. Please try again later.',
@@ -101,10 +101,12 @@ export function registerStatsRoute({
       if (isExtended) {
         const callCluster = context.core.elasticsearch.legacy.client.callAsCurrentUser;
         const esClient = context.core.elasticsearch.client.asCurrentUser;
-        const collectorsReady = await collectorSet.areAllCollectorsReady();
 
-        if (shouldGetUsage && !collectorsReady) {
-          return res.customError({ statusCode: 503, body: { message: STATS_NOT_READY_MESSAGE } });
+        if (shouldGetUsage) {
+          const collectorsReady = await collectorSet.areAllCollectorsReady();
+          if (!collectorsReady) {
+            return res.customError({ statusCode: 503, body: { message: STATS_NOT_READY_MESSAGE } });
+          }
         }
 
         const usagePromise = shouldGetUsage ? getUsage(callCluster, esClient) : Promise.resolve({});
@@ -152,9 +154,8 @@ export function registerStatsRoute({
         }
       }
 
-      // Guranteed to resolve immediately due to replay effect on getOpsMetrics$
-      // eslint-disable-next-line @typescript-eslint/naming-convention
-      const { collected_at, ...lastMetrics } = await metrics
+      // Guaranteed to resolve immediately due to replay effect on getOpsMetrics$
+      const { collected_at: collectedAt, ...lastMetrics } = await metrics
         .getOpsMetrics$()
         .pipe(first())
         .toPromise();
@@ -173,7 +174,7 @@ export function registerStatsRoute({
           snapshot: SNAPSHOT_REGEX.test(config.kibanaVersion),
           status: ServiceStatusToLegacyState[overallStatus.level.toString()],
         },
-        last_updated: collected_at.toISOString(),
+        last_updated: collectedAt.toISOString(),
         collection_interval_in_millis: metrics.collectionInterval,
       });