Add a SamplingHandler

It wraps around another handler, and only passes on some messages
based on the provided sampling handler.

This was originally written for MediaWiki: https://gerrit.wikimedia.org/r/181346

tests/Monolog/Handler/SamplingHandlerTest.php

@@ -0,0 +1,35 @@
+<?php
+
+/*
+ * This file is part of the Monolog package.
+ *
+ * (c) Jordi Boggiano <j.boggiano@seld.be>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Monolog\Handler;
+
+use Monolog\TestCase;
+use Monolog\Logger;
+
+/**
+ * @covers Monolog\Handler\SamplingHandler::handle
+ */
+class SamplingHandlerTest extends TestCase
+{
+    public function testHandle()
+    {
+        $testHandler = new TestHandler();
+        $handler = new SamplingHandler($testHandler, 2);
+        for ($i=0; $i<10000; $i++)
+        {
+            $handler->handle($this->getRecord());
+        }
+        $count = count($testHandler->getRecords());
+        // $count should be half of 10k, so between 4k and 6k
+        $this->assertLessThan(6000, $count);
+        $this->assertGreaterThan(4000, $count);
+    }
+}