[Fleet] Remove subseconds from event.ingested (#104044)

The `event.ingested` field is added to all documents ingested via
Fleet plus Agent. By removing the subseconds we can be better compression
of the values in Elasticsearch.

The primary user of `event.ingested` today is the the Security Detection Engine
as a tie-breaker in search_after, but once it moves to the using the
point-in-time API the need for precision will be lessened because PIT has
an implicit tie-breaker.

Relates #103944
Relates https://github.com/elastic/beats/issues/22388

Co-authored-by: Kibana Machine <42973632+kibanamachine@users.noreply.github.com>

x-pack/plugins/fleet/server/constants/fleet_es_assets.ts

@@ -23,6 +23,7 @@ export const FLEET_GLOBAL_COMPONENT_TEMPLATE_CONTENT = {
           properties: {
             ingested: {
               type: 'date',
+              format: 'strict_date_time_no_millis||strict_date_optional_time||epoch_millis',
             },
             agent_id_status: {
               ignore_above: 1024,
@@ -42,7 +43,12 @@ processors:
   - set:
       description: Add time when event was ingested.
       field: event.ingested
-      value: '{{{_ingest.timestamp}}}'
+      copy_from: _ingest.timestamp
+  - script:
+      description: Remove sub-seconds from event.ingested to improve storage efficiency.
+      tag: truncate-subseconds-event-ingested
+      source: ctx.event.ingested = ctx.event.ingested.withNano(0).format(DateTimeFormatter.ISO_OFFSET_DATE_TIME);
+      ignore_failure: true
   - remove:
       description: Remove any pre-existing untrusted values.
       field:

x-pack/test/fleet_api_integration/apis/epm/final_pipeline.ts

@@ -91,6 +91,26 @@ export default function (providerContext: FtrProviderContext) {
       );
     });
 
+    it('all docs should contain event.ingested without sub-seconds', async () => {
+      const res = await es.index({
+        index: 'logs-log.log-test',
+        body: {
+          '@timestamp': '2020-01-01T09:09:00',
+          message: 'hello',
+        },
+      });
+
+      const { body: doc } = await es.get({
+        id: res.body._id,
+        index: res.body._index,
+      });
+      // @ts-expect-error
+      const ingestTimestamp = doc._source.event.ingested;
+
+      // 2021-06-30T12:06:28Z
+      expect(ingestTimestamp).to.match(/^\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}Z$/);
+    });
+
     it('For a doc written without api key should write the correct api key status', async () => {
       const res = await es.index({
         index: 'logs-log.log-test',