Added Mapped Diagnostic Context (MDC) support (#2907)

* Added Mapped Diagnostic Context (MDC) support

* Update include statement

* Optimize string creation

* Fix includes

* Fix padding rules in mdc empty case

* Add comment to describe the use of mdc formatter

include/spdlog/mdc.h

@@ -0,0 +1,31 @@
+#include <spdlog/common.h>
+#include <map>
+
+namespace spdlog {
+
+class SPDLOG_API mdc {
+public:
+    static void put(const std::string &key, const std::string &value) {
+        get_context()[key] = value;
+    }
+
+    static std::string get(const std::string &key) {
+        auto &context = get_context();
+        auto it = context.find(key);
+        if (it != context.end()) {
+            return it->second;
+        }
+        return "";
+    }
+
+    static void remove(const std::string &key) { get_context().erase(key); }
+
+    static void clear() { get_context().clear(); }
+
+    static std::map<std::string, std::string> &get_context() {
+        static thread_local std::map<std::string, std::string> context;
+        return context;
+    }
+};
+
+}  // namespace spdlog

include/spdlog/pattern_formatter-inl.h

@@ -10,6 +10,7 @@
 #include <spdlog/details/fmt_helper.h>
 #include <spdlog/details/log_msg.h>
 #include <spdlog/details/os.h>
+#include <spdlog/mdc.h>
 #include <spdlog/fmt/fmt.h>
 #include <spdlog/formatter.h>
 
@@ -867,6 +868,43 @@ private:
     memory_buf_t cached_datetime_;
 };
 
+// Class for formatting Mapped Diagnostic Context (MDC) in log messages.
+// Example: [logger-name] [info] [mdc_key_1:mdc_value_1 mdc_key_2:mdc_value_2] some message
+template <typename ScopedPadder>
+class mdc_formatter : public flag_formatter {
+public:
+    explicit mdc_formatter(padding_info padinfo)
+        : flag_formatter(padinfo) {}
+
+    void format(const details::log_msg &, const std::tm &, memory_buf_t &dest) override {
+        auto mdc_map = mdc::get_context();
+        if (mdc_map.empty()) {
+            ScopedPadder p(0, padinfo_, dest);
+            return;
+        } else {
+            auto last_element = --mdc_map.end();
+            for (auto it = mdc_map.begin(); it != mdc_map.end(); ++it) {
+                auto &pair = *it;
+                const auto &key = pair.first;
+                const auto &value = pair.second;
+                size_t content_size = key.size() + value.size() + 1;  // 1 for ':'
+
+                if (it != last_element) {
+                    content_size++;  // 1 for ' '
+                }
+
+                ScopedPadder p(content_size, padinfo_, dest);
+                fmt_helper::append_string_view(key, dest);
+                fmt_helper::append_string_view(":", dest);
+                fmt_helper::append_string_view(value, dest);
+                if (it != last_element) {
+                    fmt_helper::append_string_view(" ", dest);
+                }
+            }
+        }
+    }
+};
+
 }  // namespace details
 
 SPDLOG_INLINE pattern_formatter::pattern_formatter(std::string pattern,
@@ -1159,6 +1197,10 @@ SPDLOG_INLINE void pattern_formatter::handle_flag_(char flag, details::padding_i
                     padding));
             break;
 
+        case ('&'):
+            formatters_.push_back(details::make_unique<details::mdc_formatter<Padder>>(padding));
+            break;
+
         default:  // Unknown flag appears as is
             auto unknown_flag = details::make_unique<details::aggregate_formatter>();