Unit test for very large percentile aggs (#36122)

Closes #19528

 * Port a test Colin wrote for the TDigest library to validate TDigests storing over 2*MAXINT values.  This appears to have been fixed in version 3.2 of TDigest, which Elasticsearch has been using for some time, so no changes were necessary to resolve this issue.

server/src/test/java/org/elasticsearch/search/aggregations/metrics/TDigestStateTests.java

@@ -0,0 +1,53 @@
+/*
+ * Licensed to Elasticsearch under one or more contributor
+ * license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright
+ * ownership. Elasticsearch 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.
+ */
+
+package org.elasticsearch.search.aggregations.metrics;
+
+import org.elasticsearch.test.ESTestCase;
+
+import java.util.Arrays;
+
+public class TDigestStateTests extends ESTestCase {
+
+    public void testMoreThan4BValues() {
+        // Regression test for #19528
+        // See https://github.com/tdunning/t-digest/pull/70/files#diff-4487072cee29b939694825647928f742R439
+        TDigestState digest = new TDigestState(100);
+        for (int i = 0; i < 1000; ++i) {
+            digest.add(randomDouble());
+        }
+        final int count = 1 << 29;
+        for (int i = 0; i < 10; ++i) {
+            digest.add(randomDouble(), count);
+        }
+        assertEquals(1000 + 10L * (1 << 29), digest.size());
+        assertTrue(digest.size() > 2 * Integer.MAX_VALUE);
+        final double[] quantiles = new double[] { 0, 0.1, 0.5, 0.9, 1, randomDouble()};
+        Arrays.sort(quantiles);
+        double prev = Double.NEGATIVE_INFINITY;
+        for (double q : quantiles) {
+            final double v = digest.quantile(q);
+            logger.trace("q=" + q + ", v=" + v);
+            assertTrue(v >= prev);
+            assertTrue("Unexpectedly low value: " + v, v >= 0.0);
+            assertTrue("Unexpectedly high value: " + v, v <= 1.0);
+            prev = v;
+        }
+    }
+}