feat(prometheus): add Prometheus payload and HTTP server backends

CMakeLists.txt

@@ -9,6 +9,8 @@ option(LOGIT_WITH_GZIP "Enable gzip via zlib" OFF)
 option(LOGIT_WITH_ZSTD "Enable zstd" OFF)
 option(LOGIT_WITH_FMT "Enable fmt support" OFF)
 option(LOGIT_WITH_OTLP "Enable OTLP/HTTP log export via optional kurlyk dependency" OFF)
+option(LOGIT_WITH_PROMETHEUS "Enable Prometheus text payload support" OFF)
+option(LOGIT_WITH_PROMETHEUS_SERVER "Enable Prometheus HTTP server backend" OFF)
 option(LOGIT_USE_SUBMODULES "Allow bundled optional dependency fallback" OFF)
 option(LOGIT_WITH_SYSLOG "Enable POSIX syslog backend" ON)
 option(LOGIT_WITH_WIN_EVENT_LOG "Enable Windows Event Log backend" ON)
@@ -21,7 +23,7 @@ option(LOGIT_USE_MPSC_RING "Enable lock-free TaskExecutor queue" ON)
 option(LOGIT_ENABLE_DROP_OLDEST_SLOWPATH "Enable TaskExecutor DropOldest slow-path" ON)
 
 if(NOT DEFINED CMAKE_CXX_STANDARD)
-    if(LOGIT_WITH_OTLP)
+    if(LOGIT_WITH_OTLP OR LOGIT_WITH_PROMETHEUS_SERVER)
         set(CMAKE_CXX_STANDARD 17)
     else()
         set(CMAKE_CXX_STANDARD 11)
@@ -130,6 +132,54 @@ if(LOGIT_WITH_OTLP)
     target_link_libraries(log-it-cpp INTERFACE kurlyk)
 endif()
 
+# ---------- Prometheus ----------
+if(LOGIT_WITH_PROMETHEUS)
+    if(EMSCRIPTEN)
+        message(FATAL_ERROR "LOGIT_WITH_PROMETHEUS is not supported for Emscripten.")
+    endif()
+    target_compile_definitions(log-it-cpp INTERFACE LOGIT_WITH_PROMETHEUS=1)
+endif()
+
+if(LOGIT_WITH_PROMETHEUS_SERVER)
+    if(EMSCRIPTEN)
+        message(FATAL_ERROR "LOGIT_WITH_PROMETHEUS_SERVER is not supported for Emscripten.")
+    endif()
+
+    target_compile_definitions(log-it-cpp INTERFACE ASIO_STANDALONE)
+
+    # Prefer standalone external/Simple-Web-Server
+    if(EXISTS "${CMAKE_CURRENT_SOURCE_DIR}/external/Simple-Web-Server/server_http.hpp")
+        target_include_directories(log-it-cpp INTERFACE
+            $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/external/Simple-Web-Server>
+        )
+        if(EXISTS "${CMAKE_CURRENT_SOURCE_DIR}/external/Simple-Web-Server/../asio/include/asio.hpp")
+            target_include_directories(log-it-cpp INTERFACE
+                $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/external/asio/include>
+            )
+        endif()
+    elseif(EXISTS "${CMAKE_CURRENT_SOURCE_DIR}/external/kurlyk/external/Simple-Web-Server/server_http.hpp")
+        target_include_directories(log-it-cpp INTERFACE
+            $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/external/kurlyk/external/Simple-Web-Server>
+        )
+        if(EXISTS "${CMAKE_CURRENT_SOURCE_DIR}/external/kurlyk/external/asio/include/asio.hpp")
+            target_include_directories(log-it-cpp INTERFACE
+                $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/external/kurlyk/external/asio/include>
+            )
+        endif()
+    else()
+        message(FATAL_ERROR "Simple-Web-Server not found. Add it as external/Simple-Web-Server or enable/provide kurlyk submodule.")
+    endif()
+
+    if(NOT LOGIT_WITH_PROMETHEUS)
+        set(LOGIT_WITH_PROMETHEUS ON)
+        target_compile_definitions(log-it-cpp INTERFACE LOGIT_WITH_PROMETHEUS=1)
+    endif()
+    target_compile_definitions(log-it-cpp INTERFACE LOGIT_WITH_PROMETHEUS_SERVER=1)
+    if(WIN32)
+        target_link_libraries(log-it-cpp INTERFACE ws2_32 wsock32)
+    endif()
+endif()
+
 # ---------- GZIP (zlib) ----------
 if(LOGIT_WITH_GZIP)
     if(NOT TARGET ZLIB::ZLIB)

docs/PrometheusLogger.md

@@ -0,0 +1,116 @@
+# Prometheus Logger
+
+## Overview
+
+LogIt++ provides two Prometheus backends for exposing internal log metrics in the
+[Prometheus text exposition format](https://prometheus.io/docs/instrumenting/exposition_formats/):
+
+- **PrometheusPayloadLogger** -- callback-based; delivers the serialized payload to a
+  user-provided function. Useful when you have your own HTTP server or want to push to
+  a Prometheus Pushgateway.
+
+- **PrometheusHttpServerLogger** -- embedded HTTP server; serves `/metrics` on a
+  configurable port using Simple-Web-Server. Ideal for simple services without a
+  separate metrics endpoint.
+
+## Built-in Metrics
+
+| Metric | Type | Description |
+|--------|------|-------------|
+| `logit_log_records_total` | counter | Total log records processed |
+| `logit_dropped_logs_total` | counter | Dropped log records |
+| `logit_failed_exports_total` | counter | Failed export/callback attempts |
+| `logit_last_log_timestamp_ms` | gauge | Timestamp of last log (ms) |
+| `logit_time_since_last_log_ms` | gauge | Time since last log (ms) |
+| `logit_build_info` | gauge | Build info (value=1, labels: version, compiler) |
+
+The `metric_prefix` config option (default: `logit_`) is applied to all metric names.
+
+## CMake Options
+
+```cmake
+option(LOGIT_WITH_PROMETHEUS "Enable Prometheus text payload support" OFF)
+option(LOGIT_WITH_PROMETHEUS_SERVER "Enable Prometheus HTTP server backend" OFF)
+```
+
+`LOGIT_WITH_PROMETHEUS_SERVER` implies `LOGIT_WITH_PROMETHEUS` and requires C++17
+(Simple-Web-Server dependency).
+
+## Usage: PrometheusPayloadLogger
+
+```cpp
+#include <logit.hpp>
+
+logit::PrometheusPayloadLogger::Config config;
+config.format.metric_prefix = "myapp_";
+config.emit_on_wait = true;
+config.on_payload = [](std::string payload) {
+    // Send to your HTTP endpoint or Pushgateway
+};
+
+LOGIT_ADD_LOGGER(
+    logit::PrometheusPayloadLogger,
+    (config),
+    logit::SimpleLogFormatter,
+    ("%v")
+);
+
+LOGIT_INFO("Application started");
+LOGIT_WAIT(); // triggers on_payload with current metrics
+```
+
+## Usage: PrometheusHttpServerLogger
+
+```cpp
+#include <logit.hpp>
+
+logit::PrometheusHttpServerLogger::Config config;
+config.port = 9090;
+config.path = "/metrics";
+
+LOGIT_ADD_LOGGER(
+    logit::PrometheusHttpServerLogger,
+    (config),
+    logit::SimpleLogFormatter,
+    ("%v")
+);
+
+LOGIT_INFO("Server started");
+// Scrape http://localhost:9090/metrics
+```
+
+## Custom Metrics
+
+Use the `on_collect` callback to add application-specific metrics on each scrape:
+
+```cpp
+config.on_collect = [](std::vector<logit::PrometheusMetricFamily>& families) {
+    logit::PrometheusMetricFamily mf;
+    mf.name = "myapp_queue_size";
+    mf.help = "Current queue depth";
+    mf.type = logit::PrometheusMetricType::Gauge;
+    logit::PrometheusSample s;
+    s.name = "myapp_queue_size";
+    s.value = get_queue_depth();
+    mf.samples.push_back(s);
+    families.push_back(mf);
+};
+```
+
+## Prometheus Scrape Config
+
+```yaml
+scrape_configs:
+  - job_name: 'logit-app'
+    scrape_interval: 15s
+    static_configs:
+      - targets: ['localhost:9090']
+    metrics_path: /metrics
+```
+
+## Limitations
+
+- Text exposition format only (no protobuf, no OpenMetrics `# EOF`).
+- No histograms or summaries -- use `on_collect` for custom metric types.
+- No TLS or authentication on the HTTP server.
+- No metric renaming conflicts resolution -- user must ensure unique names.

examples/example_logit_prometheus_payload.cpp

@@ -0,0 +1,35 @@
+#include <logit.hpp>
+
+int main() {
+#ifndef LOGIT_WITH_PROMETHEUS
+    LOGIT_ADD_CONSOLE_DEFAULT();
+    LOGIT_WARN("Prometheus payload example requires LOGIT_WITH_PROMETHEUS=ON");
+    LOGIT_WAIT();
+    return 0;
+#else
+    logit::PrometheusPayloadLogger::Config config;
+    config.format.metric_prefix = "myapp_";
+    config.format.include_build_info = true;
+    config.emit_on_wait = true;
+    config.on_payload = [](std::string payload) {
+        // In a real application, send payload to your Prometheus push gateway
+        // or expose it via your own HTTP endpoint.
+        (void)payload;
+    };
+
+    LOGIT_ADD_LOGGER(
+        logit::PrometheusPayloadLogger,
+        (config),
+        logit::SimpleLogFormatter,
+        ("%v")
+    );
+
+    LOGIT_INFO("Prometheus payload logger started");
+    LOGIT_WARN("Example warning message");
+    LOGIT_ERROR("Example error message");
+
+    LOGIT_WAIT();
+    LOGIT_SHUTDOWN();
+    return 0;
+#endif
+}

examples/example_logit_prometheus_server.cpp

@@ -0,0 +1,47 @@
+#include <logit.hpp>
+
+int main() {
+#ifndef LOGIT_WITH_PROMETHEUS_SERVER
+    LOGIT_ADD_CONSOLE_DEFAULT();
+    LOGIT_WARN("Prometheus server example requires LOGIT_WITH_PROMETHEUS_SERVER=ON");
+    LOGIT_WAIT();
+    return 0;
+#else
+    logit::PrometheusHttpServerLogger::Config config;
+    config.port = 9090;
+    config.path = "/metrics";
+    config.format.metric_prefix = "myapp_";
+    config.format.include_build_info = true;
+
+    // Optional: add custom metrics on each scrape
+    config.on_collect = [](std::vector<logit::PrometheusMetricFamily>& families) {
+        logit::PrometheusMetricFamily mf;
+        mf.name = "myapp_uptime_seconds";
+        mf.help = "Application uptime in seconds";
+        mf.type = logit::PrometheusMetricType::Gauge;
+        logit::PrometheusSample s;
+        s.name = "myapp_uptime_seconds";
+        s.value = 42.0;
+        mf.samples.push_back(s);
+        families.push_back(mf);
+    };
+
+    LOGIT_ADD_LOGGER(
+        logit::PrometheusHttpServerLogger,
+        (config),
+        logit::SimpleLogFormatter,
+        ("%v")
+    );
+
+    LOGIT_INFO("Prometheus HTTP server started on port 9090");
+    LOGIT_WARN("Scrape metrics at http://localhost:9090/metrics");
+
+    for (int i = 0; i < 5; ++i) {
+        LOGIT_INFO("Logging iteration %d", (i + 1));
+        std::this_thread::sleep_for(std::chrono::seconds(1));
+    }
+
+    LOGIT_SHUTDOWN();
+    return 0;
+#endif
+}

include/logit_cpp/logit/loggers.hpp

@@ -33,4 +33,11 @@
 #include "loggers/OtlpPayloadLogger.hpp"
 #endif
 
+#ifdef LOGIT_WITH_PROMETHEUS
+#include "loggers/PrometheusPayloadLogger.hpp"
+#endif
+#ifdef LOGIT_WITH_PROMETHEUS_SERVER
+#include "loggers/PrometheusHttpServerLogger.hpp"
+#endif
+
 #endif // _LOGIT_LOGGERS_HPP_INCLUDED