Add ClosureContextProcessor (#1745)

Co-authored-by: Jordi Boggiano <j.boggiano@seld.be>

src/Monolog/Processor/ClosureContextProcessor.php

@@ -0,0 +1,51 @@
+<?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\LogRecord;
+
+/**
+ * Generates a context from a Closure if the Closure is the only value
+ * in the context
+ *
+ * It helps reduce the performance impact of debug logs if they do
+ * need to create lots of context information. If this processor is added
+ * on the correct handler the context data will only be generated
+ * when the logs are actually logged to that handler, which is useful when
+ * using FingersCrossedHandler or other filtering handlers to conditionally
+ * log records.
+ */
+class ClosureContextProcessor implements ProcessorInterface
+{
+    public function __invoke(LogRecord $record): LogRecord
+    {
+        $context = $record->context;
+        if (isset($context[0]) && 1 === \count($context) && $context[0] instanceof \Closure) {
+            try {
+                $context = $context[0]();
+            } catch (\Throwable $e) {
+                $context = [
+                    'error_on_context_generation' => $e->getMessage(),
+                    'exception' => $e,
+                ];
+            }
+
+            if (!\is_array($context)) {
+                $context = [$context];
+            }
+
+            $record = $record->with(context: $context);
+        }
+
+        return $record;
+    }
+}