Add Cookbook S01-S22 (draft)

docs-src/pages/en/cookbook/s15-server-logger.md

@@ -0,0 +1,64 @@
+---
+title: "S15. Log Requests on the Server"
+order: 34
+status: "draft"
+---
+
+To log the requests the server receives and the responses it returns, use `Server::set_logger()`. The callback fires once per completed request, making it the foundation for access logs and metrics collection.
+
+## Basic usage
+
+```cpp
+svr.set_logger([](const httplib::Request &req, const httplib::Response &res) {
+  std::cout << req.remote_addr << " "
+            << req.method << " " << req.path
+            << " -> " << res.status << std::endl;
+});
+```
+
+The log callback receives both the `Request` and the `Response`. You can grab the method, path, status, client IP, headers, body — whatever you need.
+
+## Access-log style format
+
+Here's an Apache/Nginx-ish access log format.
+
+```cpp
+svr.set_logger([](const auto &req, const auto &res) {
+  auto now = std::time(nullptr);
+  char timebuf[32];
+  std::strftime(timebuf, sizeof(timebuf), "%Y-%m-%d %H:%M:%S",
+                std::localtime(&now));
+
+  std::cout << timebuf << " "
+            << req.remote_addr << " "
+            << "\"" << req.method << " " << req.path << "\" "
+            << res.status << " "
+            << res.body.size() << "B"
+            << std::endl;
+});
+```
+
+## Measure request time
+
+To include request duration in the log, stash a start timestamp in `res.user_data` from a pre-routing handler, then subtract in the logger.
+
+```cpp
+svr.set_pre_routing_handler([](const auto &req, auto &res) {
+  res.user_data.set("start", std::chrono::steady_clock::now());
+  return httplib::Server::HandlerResponse::Unhandled;
+});
+
+svr.set_logger([](const auto &req, const auto &res) {
+  auto *start = res.user_data.get<std::chrono::steady_clock::time_point>("start");
+  auto elapsed = start
+    ? std::chrono::duration_cast<std::chrono::milliseconds>(
+        std::chrono::steady_clock::now() - *start).count()
+    : 0;
+  std::cout << req.method << " " << req.path
+            << " " << res.status << " " << elapsed << "ms" << std::endl;
+});
+```
+
+For more on `user_data`, see S12. Pass Data Between Handlers with `res.user_data`.
+
+> **Note:** The logger runs synchronously on the same thread as request processing. Heavy work inside it hurts throughput — push it to a queue and process asynchronously if you need anything expensive.