Add Cookbook C01-C19 (draft)

docs-src/pages/en/cookbook/c19-client-logger.md

@@ -0,0 +1,57 @@
+---
+title: "C19. Set a Logger on the Client"
+order: 19
+status: "draft"
+---
+
+To log requests sent and responses received by the client, use `set_logger()`. If you only care about errors, there's a separate `set_error_logger()`.
+
+## Log requests and responses
+
+```cpp
+httplib::Client cli("https://api.example.com");
+
+cli.set_logger([](const httplib::Request &req, const httplib::Response &res) {
+  std::cout << req.method << " " << req.path
+            << " -> " << res.status << std::endl;
+});
+
+auto res = cli.Get("/users");
+```
+
+The callback you pass to `set_logger()` fires once for each completed request. You get both the request and the response as arguments — so you can log the method, path, status, headers, body, or whatever else you need.
+
+## Catch errors only
+
+When a network-layer error happens (like `Error::Connection`), `set_logger()` is **not** called — there's no response to log. For those cases, use `set_error_logger()`.
+
+```cpp
+cli.set_error_logger([](const httplib::Error &err, const httplib::Request *req) {
+  std::cerr << "error: " << httplib::to_string(err);
+  if (req) {
+    std::cerr << " (" << req->method << " " << req->path << ")";
+  }
+  std::cerr << std::endl;
+});
+```
+
+The second argument `req` can be null — it happens when the failure occurred before the request was built. Always null-check before dereferencing.
+
+## Use both together
+
+A nice pattern is to log successes through one, failures through the other.
+
+```cpp
+cli.set_logger([](const auto &req, const auto &res) {
+  std::cout << "[ok] " << req.method << " " << req.path
+            << " " << res.status << std::endl;
+});
+
+cli.set_error_logger([](const auto &err, const auto *req) {
+  std::cerr << "[ng] " << httplib::to_string(err);
+  if (req) std::cerr << " " << req->method << " " << req->path;
+  std::cerr << std::endl;
+});
+```
+
+> **Note:** The log callbacks run synchronously on the same thread as the request. Heavy work inside them slows the request down — push it to a background queue if you need to do anything expensive.