use custom logger for promhttp

Author: idoqo

exporter/exporter.go

@@ -363,7 +363,7 @@ func (e *Exporter) Handler() http.Handler {
 		// Delegate http serving to Prometheus client library, which will call collector.Collect.
 		h := promhttp.HandlerFor(gatherers, promhttp.HandlerOpts{
 			ErrorHandling: promhttp.ContinueOnError,
-			// ErrorLog:      e.logger, // todo(idoqo): check with prometheus/client_golang about the interface here
+			ErrorLog:      newHTTPErrorLogger(e.logger),
 		})
 
 		h.ServeHTTP(w, r)

exporter/http_error_logger.go

@@ -0,0 +1,38 @@
+package exporter
+
+import (
+	"fmt"
+	"github.com/pkg/errors"
+	"github.com/prometheus/client_golang/prometheus"
+	"log/slog"
+)
+
+// httpErrorLogger is a wrapper around slog.Logger that can log promhttp errors (by implementing a Println method).
+type httpErrorLogger struct {
+	logger *slog.Logger
+}
+
+func newHTTPErrorLogger(logger *slog.Logger) *httpErrorLogger {
+	return &httpErrorLogger{logger: logger}
+}
+
+// Println implements the Println method for httpErrorHandler.
+func (h *httpErrorLogger) Println(v ...any) {
+	// promhttp calls the Println() method as follows:
+	// logger.Println(message, err) i.e., v[0] is the message (a string) and v[1] is the error (which might be prometheus.MultiError)
+	if len(v) == 2 {
+		msg, mok := v[0].(string)
+		err, eok := v[1].(error)
+		if mok && eok {
+			multiErr := prometheus.MultiError{}
+			if errors.As(err, &multiErr) {
+				errCount := len(multiErr)
+				for i, err := range multiErr {
+					h.logger.Error(msg, "error", err, "total_errors", errCount, "error_no", i)
+				}
+			}
+		}
+	}
+	// fallback
+	h.logger.Error(fmt.Sprint(v...))
+}

exporter/server.go

@@ -170,7 +170,7 @@ func OverallTargetsHandler(exporters []*Exporter, logger *slog.Logger) http.Hand
 		// Delegate http serving to Prometheus client library, which will call collector.Collect.
 		h := promhttp.HandlerFor(gatherers, promhttp.HandlerOpts{
 			ErrorHandling: promhttp.ContinueOnError,
-			// ErrorLog:      logger, // todo(idoqo): check with promhttp
+			ErrorLog:      newHTTPErrorLogger(logger),
 		})
 
 		h.ServeHTTP(w, r)