[ML] [AIOps] Log Rate Analysis: Show top N results when no docs in ba…

…seline or deviation. (#171924)

This PR adds a fallback analysis when either the baseline or deviation
time range contains no documents. Instead of trying to fetch significant
items (which would return no results), this will just fetch the top
terms/categories for the time range that has some documents.

x-pack/plugins/aiops/common/__mocks__/artificial_logs/top_terms.ts

@@ -0,0 +1,142 @@
+/*
+ * 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 type { SignificantItem } from '@kbn/ml-agg-utils';
+
+// Named topTerms since all these items are of type `keyword`.
+export const topTerms: SignificantItem[] = [
+  {
+    bg_count: 0,
+    doc_count: 5102,
+    fieldName: 'version',
+    fieldValue: 'v1.0.0',
+    key: 'version:v1.0.0',
+    normalizedScore: 0,
+    pValue: 1,
+    score: 0,
+    total_bg_count: 0,
+    total_doc_count: 0,
+    type: 'keyword',
+  },
+  {
+    bg_count: 0,
+    doc_count: 2272,
+    fieldName: 'response_code',
+    fieldValue: '500',
+    key: 'response_code:500',
+    normalizedScore: 0,
+    pValue: 1,
+    score: 0,
+    total_bg_count: 0,
+    total_doc_count: 0,
+    type: 'keyword',
+  },
+  {
+    bg_count: 0,
+    doc_count: 2197,
+    fieldName: 'url',
+    fieldValue: 'home.php',
+    key: 'url:home.php',
+    normalizedScore: 0,
+    pValue: 1,
+    score: 0,
+    total_bg_count: 0,
+    total_doc_count: 0,
+    type: 'keyword',
+  },
+  {
+    bg_count: 0,
+    doc_count: 1981,
+    fieldName: 'user',
+    fieldValue: 'Peter',
+    key: 'user:Peter',
+    normalizedScore: 0,
+    pValue: 1,
+    score: 0,
+    total_bg_count: 0,
+    total_doc_count: 0,
+    type: 'keyword',
+  },
+  {
+    bg_count: 0,
+    doc_count: 1773,
+    fieldName: 'user',
+    fieldValue: 'Paul',
+    key: 'user:Paul',
+    normalizedScore: 0,
+    pValue: 1,
+    score: 0,
+    total_bg_count: 0,
+    total_doc_count: 0,
+    type: 'keyword',
+  },
+  {
+    bg_count: 0,
+    doc_count: 1574,
+    fieldName: 'url',
+    fieldValue: 'login.php',
+    key: 'url:login.php',
+    normalizedScore: 0,
+    pValue: 1,
+    score: 0,
+    total_bg_count: 0,
+    total_doc_count: 0,
+    type: 'keyword',
+  },
+  {
+    bg_count: 0,
+    doc_count: 1569,
+    fieldName: 'response_code',
+    fieldValue: '404',
+    key: 'response_code:404',
+    normalizedScore: 0,
+    pValue: 1,
+    score: 0,
+    total_bg_count: 0,
+    total_doc_count: 0,
+    type: 'keyword',
+  },
+  {
+    bg_count: 0,
+    doc_count: 1348,
+    fieldName: 'user',
+    fieldValue: 'Mary',
+    key: 'user:Mary',
+    normalizedScore: 0,
+    pValue: 1,
+    score: 0,
+    total_bg_count: 0,
+    total_doc_count: 0,
+    type: 'keyword',
+  },
+  {
+    bg_count: 0,
+    doc_count: 1331,
+    fieldName: 'url',
+    fieldValue: 'user.php',
+    key: 'url:user.php',
+    normalizedScore: 0,
+    pValue: 1,
+    score: 0,
+    total_bg_count: 0,
+    total_doc_count: 0,
+    type: 'keyword',
+  },
+  {
+    bg_count: 0,
+    doc_count: 1261,
+    fieldName: 'response_code',
+    fieldValue: '200',
+    key: 'response_code:200',
+    normalizedScore: 0,
+    pValue: 1,
+    score: 0,
+    total_bg_count: 0,
+    total_doc_count: 0,
+    type: 'keyword',
+  },
+];