Add LoadAverageProcessor (#1803)

Co-authored-by: johan Vlaar <johan@adivare.nl>
Co-authored-by: Jordi Boggiano <j.boggiano@seld.be>

tests/Monolog/Processor/LoadAverageProcessorTest.php

@@ -0,0 +1,38 @@
+<?php declare(strict_types=1);
+
+/*
+ * 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\Processor;
+
+use Monolog\Test\TestCase;
+
+class LoadAverageProcessorTest extends TestCase
+{
+    /**
+     * @covers Monolog\Processor\LoadAverageProcessor::__invoke
+     */
+    public function testProcessor()
+    {
+        $processor = new LoadAverageProcessor();
+        $record = $processor($this->getRecord());
+        $this->assertArrayHasKey('load_average', $record->extra);
+        $this->assertIsFloat($record->extra['load_average']);
+    }
+
+    /**
+     * @covers Monolog\Processor\LoadAverageProcessor::__invoke
+     */
+    public function testProcessorWithInvalidAvgSystemLoad()
+    {
+        $this->expectException(\InvalidArgumentException::class);
+        $this->expectExceptionMessage('Invalid average system load: `3`');
+        new LoadAverageProcessor(3);
+    }
+}