[#375] Add log metrics

Signed-off-by: Alejandro Lopez <a.lopez@yadro.com>

pkg/util/logger/metrics.go

@@ -0,0 +1,37 @@
+package logger
+
+import (
+	"strconv"
+
+	"git.frostfs.info/TrueCloudLab/frostfs-observability/metrics"
+	"github.com/prometheus/client_golang/prometheus"
+	"go.uber.org/zap/zapcore"
+)
+
+const (
+	logSubsystem    = "log"
+	logLevelLabel   = "level"
+	logDroppedLabel = "dropped"
+)
+
+type logMetrics struct {
+	logCount *prometheus.CounterVec
+}
+
+func newLogMetrics(namespace string) *logMetrics {
+	return &logMetrics{
+		logCount: metrics.NewCounterVec(prometheus.CounterOpts{
+			Namespace: namespace,
+			Subsystem: logSubsystem,
+			Name:      "log_count",
+			Help:      "Total log entries emitted or dropped by severity level",
+		}, []string{logLevelLabel, logDroppedLabel}),
+	}
+}
+
+func (m *logMetrics) Inc(level zapcore.Level, dropped bool) {
+	m.logCount.With(prometheus.Labels{
+		logLevelLabel:   level.String(),
+		logDroppedLabel: strconv.FormatBool(dropped),
+	}).Inc()
+}