Address precision issue in IndexDiskUsageAnalyzerTests#testCompletionFields (#125849) (#125951) (#125963)

We have some tolerance wound how many bytes we report for these completion fields. But the
values depend on the distribution of the random values that determine how many docs get
an option field. This commit makes the test more precise by computing the real ratio
between docs that have the optional field and the total number of docs, so that we
can base assertion on more realistic expectations.

Closes #123269

Author: javanna

server/src/test/java/org/elasticsearch/action/admin/indices/diskusage/IndexDiskUsageAnalyzerTests.java

@@ -339,23 +339,27 @@ public PostingsFormat getPostingsFormatForField(String field) {
             });
 
         try (Directory dir = createNewDirectory()) {
+            float docsWithSuggest1FieldRatio;
             try (IndexWriter writer = new IndexWriter(dir, config)) {
                 int numDocs = randomIntBetween(100, 1000);
+                int numDocsWithSuggest1Field = 0;
                 for (int i = 0; i < numDocs; i++) {
                     final Document doc = new Document();
                     if (randomDouble() < 0.5) {
+                        numDocsWithSuggest1Field++;
                         doc.add(new SuggestField("suggest_1", randomAlphaOfLength(10), randomIntBetween(1, 20)));
                     }
                     doc.add(new SuggestField("suggest_2", randomAlphaOfLength(10), randomIntBetween(1, 20)));
                     writer.addDocument(doc);
                 }
+                docsWithSuggest1FieldRatio = (float) numDocsWithSuggest1Field / (numDocs + numDocsWithSuggest1Field);
             }
             final IndexDiskUsageStats stats = IndexDiskUsageAnalyzer.analyze(testShardId(), lastCommit(dir), () -> {});
             assertFieldStats(
                 "suggest_1",
                 "inverted_index",
                 stats.getFields().get("suggest_1").getInvertedIndexBytes(),
-                stats.total().totalBytes() / 3,
+                (long) (stats.total().totalBytes() * docsWithSuggest1FieldRatio),
                 0.05,
                 2048
             );
@@ -364,7 +368,7 @@ public PostingsFormat getPostingsFormatForField(String field) {
                 "suggest_2",
                 "inverted_index",
                 stats.getFields().get("suggest_2").getInvertedIndexBytes(),
-                stats.total().totalBytes() * 2 / 3,
+                (long) (stats.total().totalBytes() * (1 - docsWithSuggest1FieldRatio)),
                 0.05,
                 2048
             );