[Infra] apm_data_access service to return host names (elastic#189894)

closes [elastic#188752](elastic#188752)

## Summary

This PR creates a service that will return host names collected by APM. 

**NOTE**: At first, we'll use this service to join the host names from
APM with metrics collected by the system module to build the hosts with
metrics lists - not necessarily to be used in the total hosts count
functionality. This service may be adjusted as we start using it in
Infra.

---------

Co-authored-by: Elastic Machine <elasticmachine@users.noreply.github.com>

x-pack/plugins/observability_solution/apm_data_access/server/services/get_host_names/index.ts

@@ -0,0 +1,66 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the Elastic License
+ * 2.0; you may not use this file except in compliance with the Elastic License
+ * 2.0.
+ */
+
+import { estypes } from '@elastic/elasticsearch';
+import { rangeQuery } from '@kbn/observability-plugin/server';
+import { HOST_NAME } from '@kbn/apm-types/es_fields';
+import { getBucketSize, type TimeRangeMetadata } from '../../../common';
+import { getPreferredBucketSizeAndDataSource } from '../../../common/utils/get_preferred_bucket_size_and_data_source';
+import { ApmDocumentType } from '../../../common/document_type';
+import type { ApmDataAccessServicesParams } from '../get_services';
+
+const MAX_SIZE = 1000;
+
+export interface HostNamesRequest {
+  query: estypes.QueryDslQueryContainer;
+  kuery?: string;
+  start: number;
+  end: number;
+  size?: number;
+  documentSources: TimeRangeMetadata['sources'];
+}
+
+const suitableTypes = [ApmDocumentType.TransactionMetric];
+
+export function createGetHostNames({ apmEventClient }: ApmDataAccessServicesParams) {
+  return async ({ start, end, size = MAX_SIZE, query, documentSources }: HostNamesRequest) => {
+    const sourcesToUse = getPreferredBucketSizeAndDataSource({
+      sources: documentSources.filter((s) => suitableTypes.includes(s.documentType)),
+      bucketSizeInSeconds: getBucketSize({ start, end, numBuckets: 50 }).bucketSize,
+    });
+
+    const esResponse = await apmEventClient.search('get_apm_host_names', {
+      apm: {
+        sources: [
+          {
+            documentType: sourcesToUse.source.documentType,
+            rollupInterval: sourcesToUse.source.rollupInterval,
+          },
+        ],
+      },
+      body: {
+        track_total_hits: false,
+        size: 0,
+        query: {
+          bool: {
+            filter: [query, ...rangeQuery(start, end)],
+          },
+        },
+        aggs: {
+          hostNames: {
+            terms: {
+              field: HOST_NAME,
+              size: Math.min(size, MAX_SIZE),
+            },
+          },
+        },
+      },
+    });
+
+    return esResponse.aggregations?.hostNames.buckets.map((bucket) => bucket.key as string) ?? [];
+  };
+}

x-pack/plugins/observability_solution/apm_data_access/server/services/get_services.ts

@@ -7,6 +7,7 @@
 
 import { APMEventClient } from '../lib/helpers/create_es_client/create_apm_event_client';
 import { createGetDocumentSources } from './get_document_sources';
+import { createGetHostNames } from './get_host_names';
 
 export interface ApmDataAccessServicesParams {
   apmEventClient: APMEventClient;
@@ -15,5 +16,6 @@ export interface ApmDataAccessServicesParams {
 export function getServices(params: ApmDataAccessServicesParams) {
   return {
     getDocumentSources: createGetDocumentSources(params),
+    getHostNames: createGetHostNames(params),
   };
 }

x-pack/plugins/observability_solution/apm_data_access/server/types.ts

@@ -21,6 +21,7 @@ export interface ApmDataAccessPluginStart {}
 export type ApmDataAccessServices = ReturnType<typeof getServices>;
 export type { ApmDataAccessServicesParams } from './services/get_services';
 export type { DocumentSourcesRequest } from './services/get_document_sources';
+export type { HostNamesRequest } from './services/get_host_names';
 export type {
   APMEventClientConfig,
   APMEventESSearchRequest,