feat(prometheus): add Prometheus payload and HTTP server backends - #104
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Add PrometheusPayloadLogger (callback-based text exposition) and PrometheusHttpServerLogger (embedded /metrics server via Simple-Web-Server). Includes shared text serializer, built-in logit_* counters/gauges, custom on_collect hook, and full test coverage. Constraint: Prometheus text format 0.0.4 only; no protobuf, OpenMetrics, histograms, or TLS in this PR. Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
Move LOGIT_WITH_PROMETHEUS options to top of CMakeLists.txt before C++ standard logic that depends on them. Remove unused Config fields `async` and `export_interval_ms`. Change default resource handler to return 404 for unknown paths. Add optional `/health` endpoint. Guard on_collect exception with ++m_failed_exports. Add HTTP integration tests for /metrics, /health, unknown paths, and custom paths using SimpleWeb::Client<SimpleWeb::HTTP>. Constraint: header-only library, no external test framework Rejected: custom HTTP client code | use existing Simple-Web-Server client Confidence: high Scope-risk: narrow
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Summary
Adds two Prometheus backend loggers for exposing internal log metrics in Prometheus text exposition format 0.0.4:
on_payloadcallback. Supports custom metrics viaon_collecthook./metricsendpoint on a configurable port.What's included
New headers
include/logit_cpp/logit/loggers/prometheus/PrometheusTextFormatConfig.hpp— shared types (PrometheusMetricType,PrometheusMetricFamily,PrometheusSample, config struct)include/logit_cpp/logit/loggers/prometheus/PrometheusTextSerializer.hpp— header-only serializer with escaping, sanitization, NaN/Inf handlinginclude/logit_cpp/logit/loggers/PrometheusPayloadLogger.hpp—ILogger-based callback backendinclude/logit_cpp/logit/loggers/PrometheusHttpServerLogger.hpp—ILogger-based HTTP server backendBuilt-in metrics (all prefixed with
metric_prefix, defaultlogit_)log_records_totaldropped_logs_totalfailed_exports_totallast_log_timestamp_mstime_since_last_log_msbuild_infoCMake options
LOGIT_WITH_PROMETHEUS_SERVERimpliesLOGIT_WITH_PROMETHEUS, requires C++17, and auto-discovers Simple-Web-Server (standalone or via kurlyk submodule).Tests
tests/prometheus_text_serializer_test.cpp— 13 tests (escaping, sanitization, HELP/TYPE, labels, timestamps, NaN/Inf, precision)tests/prometheus_payload_logger_test.cpp— 8 tests (collect, callback, exceptions, custom metrics, params)tests/prometheus_http_server_logger_test.cpp— 6 tests (lifecycle, payload, custom metrics, params, start/stop)Docs & examples
docs/PrometheusLogger.md— usage guide with scrape config exampleexamples/example_logit_prometheus_payload.cppexamples/example_logit_prometheus_server.cppTest plan
prometheus_text_serializer_testpassesprometheus_payload_logger_testpassesprometheus_http_server_logger_testpassesLOGIT_WITH_PROMETHEUS=ON) compiles and tests passLOGIT_WITH_PROMETHEUS_SERVER=ON) compiles and tests passScope limits (not in this PR)
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