From 2e9464f07ffbff25737fa0df413b4ee35eb45bf5 Mon Sep 17 00:00:00 2001
From: Francesco Casula <fra.casula@gmail.com>
Date: Wed, 25 Sep 2024 09:54:51 +0200
Subject: [PATCH 1/2] chore: gauges optimization

---
 stats/otel.go             | 68 +++++++++++++++++++++++++--------------
 stats/otel_measurement.go |  3 +-
 2 files changed, 45 insertions(+), 26 deletions(-)

diff --git a/stats/otel.go b/stats/otel.go
index 6a762f3e..9f192645 100644
--- a/stats/otel.go
+++ b/stats/otel.go
@@ -40,16 +40,18 @@ type otelStats struct {
 	tracerMap           map[string]Tracer
 	tracerMapMu         sync.Mutex
 
-	meter        metric.Meter
-	noopMeter    metric.Meter
-	counters     map[string]metric.Int64Counter
-	countersMu   sync.Mutex
-	gauges       map[string]*otelGauge
-	gaugesMu     sync.Mutex
-	timers       map[string]metric.Float64Histogram
-	timersMu     sync.Mutex
-	histograms   map[string]metric.Float64Histogram
-	histogramsMu sync.Mutex
+	meter              metric.Meter
+	noopMeter          metric.Meter
+	counters           map[string]metric.Int64Counter
+	countersMu         sync.Mutex
+	gauges             map[string]*otelGauge
+	gaugesOtel         map[string]metric.Float64ObservableGauge
+	gaugesMu           sync.RWMutex
+	gaugesRegisterOnce sync.Once
+	timers             map[string]metric.Float64Histogram
+	timersMu           sync.Mutex
+	histograms         map[string]metric.Float64Histogram
+	histogramsMu       sync.Mutex
 
 	otelManager              otel.Manager
 	collectorAggregator      *aggregatedCollector
@@ -385,29 +387,45 @@ func (s *otelStats) getGauge(
 	}
 
 	if !ok {
+		g, exists := s.gaugesOtel[name]
+		if !exists {
+			var err error
+			g, err = s.meter.Float64ObservableGauge(name)
+			if err != nil {
+				s.logger.Warnf("failed to create observable gauge %s: %v", name, err)
+				g, _ = s.noopMeter.Float64ObservableGauge(name)
+			}
+		}
+
 		og = &otelGauge{otelMeasurement: &otelMeasurement{
 			genericMeasurement: genericMeasurement{statType: GaugeType},
 			attributes:         attributes,
-		}}
+		}, observableGauge: g}
 
-		g, err := s.meter.Float64ObservableGauge(name)
-		if err != nil {
-			s.logger.Warnf("failed to create gauge %s: %v", name, err)
-			g, _ = s.noopMeter.Float64ObservableGauge(name)
-		} else {
-			_, err = s.meter.RegisterCallback(func(ctx context.Context, o metric.Observer) error {
+		s.gauges[mapKey] = og
+		s.gaugesOtel[name] = g
+	}
+
+	s.gaugesRegisterOnce.Do(func() {
+		_, err := s.meter.RegisterCallback(func(ctx context.Context, o metric.Observer) error {
+			s.gaugesMu.RLock()
+			defer s.gaugesMu.RUnlock()
+
+			for _, og := range s.gauges {
+				if og.disabled {
+					continue
+				}
 				if value := og.getValue(); value != nil {
-					o.ObserveFloat64(g, cast.ToFloat64(value), metric.WithAttributes(attributes...))
+					o.ObserveFloat64(og.observableGauge, cast.ToFloat64(value), metric.WithAttributes(og.attributes...))
 				}
-				return nil
-			}, g)
-			if err != nil {
-				panic(fmt.Errorf("failed to register callback for gauge %s: %w", name, err))
 			}
-		}
 
-		s.gauges[mapKey] = og
-	}
+			return nil
+		})
+		if err != nil {
+			panic(fmt.Errorf("failed to register callback for gauge %s: %w", name, err))
+		}
+	})
 
 	return og
 }
diff --git a/stats/otel_measurement.go b/stats/otel_measurement.go
index b941db53..05909a3e 100644
--- a/stats/otel_measurement.go
+++ b/stats/otel_measurement.go
@@ -38,7 +38,8 @@ func (c *otelCounter) Increment() {
 // otelGauge represents a gauge stat
 type otelGauge struct {
 	*otelMeasurement
-	value atomic.Value
+	value           atomic.Value
+	observableGauge metric.Float64ObservableGauge
 }
 
 // Gauge records an absolute value for this stat. Only applies to GaugeType stats

From 2d51ad1d2f4363c6f3a6c642c1379a3365d24fef Mon Sep 17 00:00:00 2001
From: Francesco Casula <fra.casula@gmail.com>
Date: Wed, 25 Sep 2024 09:59:21 +0200
Subject: [PATCH 2/2] fix: nil map

---
 stats/otel.go | 1 +
 1 file changed, 1 insertion(+)

diff --git a/stats/otel.go b/stats/otel.go
index 9f192645..6661943b 100644
--- a/stats/otel.go
+++ b/stats/otel.go
@@ -382,6 +382,7 @@ func (s *otelStats) getGauge(
 
 	if s.gauges == nil {
 		s.gauges = make(map[string]*otelGauge)
+		s.gaugesOtel = make(map[string]metric.Float64ObservableGauge)
 	} else {
 		og, ok = s.gauges[mapKey]
 	}