Add basic support to Google Cloud Logging format (#1719)

Google Cloud Logging doesn't show the correct level log when using
JsonFormatter, making observability a bit trickier. This applies minor
tweaks to the default format, allowing log entries to be properly
represented.

There are alternative packages to this but they add fields that aren't
strictly required - also performing `debug_backtrace()` calls that are
usually not desired when in production mode.

This is a backport of https://github.com/Seldaek/monolog/pull/1690

Signed-off-by: Luís Cobucci <lcobucci@gmail.com>

src/Monolog/Formatter/GoogleCloudLoggingFormatter.php

@@ -0,0 +1,39 @@
+<?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\Formatter;
+
+use DateTimeInterface;
+use Monolog\LogRecord;
+
+/**
+ * Encodes message information into JSON in a format compatible with Cloud logging.
+ *
+ * @see https://cloud.google.com/logging/docs/reference/v2/rest/v2/LogEntry
+ *
+ * @author Luís Cobucci <lcobucci@gmail.com>
+ */
+final class GoogleCloudLoggingFormatter extends JsonFormatter
+{
+    /** {@inheritdoc} **/
+    public function format(array $record): string
+    {
+        // Re-key level for GCP logging
+        $record['severity'] = $record['level_name'];
+        $record['timestamp'] = $record['datetime']->format(DateTimeInterface::RFC3339_EXTENDED);
+
+        // Remove keys that are not used by GCP
+        unset($record['level'], $record['level_name'], $record['datetime']);
+
+        return parent::format($record);
+    }
+}
+

tests/Monolog/Formatter/GoogleCloudLoggingFormatterTest.php

@@ -0,0 +1,54 @@
+<?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\Formatter;
+
+use DateTimeInterface;
+use Monolog\Test\TestCase;
+use function json_decode;
+
+class GoogleCloudLoggingFormatterTest extends TestCase
+{
+    /**
+     * @test
+     *
+     * @covers \Monolog\Formatter\JsonFormatter
+     * @covers \Monolog\Formatter\GoogleCloudLoggingFormatter::format
+     */
+    public function formatProvidesRfc3339Timestamps(): void
+    {
+        $formatter = new GoogleCloudLoggingFormatter();
+        $record = $this->getRecord();
+
+        $formatted_decoded = json_decode($formatter->format($record), true);
+        $this->assertArrayNotHasKey("datetime", $formatted_decoded);
+        $this->assertArrayHasKey("timestamp", $formatted_decoded);
+        $this->assertSame($record['datetime']->format(DateTimeInterface::RFC3339_EXTENDED), $formatted_decoded["timestamp"]);
+    }
+
+    /**
+     * @test
+     *
+     * @covers \Monolog\Formatter\JsonFormatter
+     * @covers \Monolog\Formatter\GoogleCloudLoggingFormatter::format
+     */
+    public function formatIntroducesLogSeverity(): void
+    {
+        $formatter = new GoogleCloudLoggingFormatter();
+        $record = $this->getRecord();
+
+        $formatted_decoded = json_decode($formatter->format($record), true);
+        $this->assertArrayNotHasKey("level", $formatted_decoded);
+        $this->assertArrayNotHasKey("level_name", $formatted_decoded);
+        $this->assertArrayHasKey("severity", $formatted_decoded);
+        $this->assertSame($record['level_name'], $formatted_decoded["severity"]);
+    }
+}