Prometheus Logger

Prometheus Logger

Overview

LogIt++ provides two Prometheus backends for exposing internal log metrics in the Prometheus text exposition format:

• PrometheusPayloadLogger – callback-based; delivers the serialized payload to a user-provided function. It can be integrated with your own HTTP server, Pushgateway client, or another existing transport.
• 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 built-in logger metric names.

PrometheusHttpServerLogger also exposes scrape diagnostics:

Metric	Type	Description
logit_prometheus_scrapes_total	counter	Total /metrics scrape requests
logit_prometheus_scrape_errors_total	counter	Failed scrape requests
logit_prometheus_last_scrape_timestamp_ms	gauge	Timestamp of the last scrape request
logit_prometheus_collect_duration_seconds	gauge	Duration of the last metrics collection

CMake Options

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).

LOGIT_WITH_PROMETHEUS_SERVER=ON currently supports source-tree and build-tree consumption only. The install/export step intentionally rejects this configuration because the Simple-Web-Server and Asio include trees are not exported package dependencies yet.

Usage: PrometheusPayloadLogger

For a runnable callback example with custom application metrics, see examples/example_logit_prometheus_payload.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

For a runnable embedded /metrics server example, see examples/example_logit_prometheus_server.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 PrometheusRegistry with the on_collect callback to register application-specific metrics once and collect them on each scrape:

#include <logit.hpp>
 
logit::PrometheusRegistry registry("myapp_");
 
registry.set_gauge(
    "queue_size",
    "Current queue depth",
    []() { return get_queue_depth(); });
 
config.on_collect = [&registry](std::vector<logit::PrometheusMetricFamily>& families) {
    registry.collect(families);
};

PrometheusTextFormatConfig::metric_prefix applies only to LogIt++ built-in metrics. Custom metric names are written as supplied by the registry or manual builders, so use the registry prefix for application metric namespaces.

If a registry value callback throws, the registry skips that sample, keeps collecting later metrics, appends the healthy samples, and then rethrows the first exception. The Prometheus loggers catch that exception from on_collect and increment their failed export counter.

For low-level control, on_collect can still append PrometheusMetricFamily objects directly or use helpers such as add_prometheus_gauge().

Prometheus Scrape Config

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.