metrics: make gauge_float64 and counter_float64 lock free (#27025)

Makes the float-gauges lock-free

name                      old time/op  new time/op  delta
CounterFloat64Parallel-8  1.45µs ±10%  0.85µs ± 6%  -41.65%  (p=0.008 n=5+5)

---------
Co-authored-by: Exca-DK <dev@DESKTOP-RI45P4J.localdomain>
Co-authored-by: Martin Holst Swende <martin@swende.se>

metrics/counter_float64.go

 package metrics
 
 import (
-	"sync"
+	"math"
+	"sync/atomic"
 )
 
 // CounterFloat64 holds a float64 value that can be incremented and decremented.
@@ -38,13 +39,13 @@ func NewCounterFloat64() CounterFloat64 {
 	if !Enabled {
 		return NilCounterFloat64{}
 	}
-	return &StandardCounterFloat64{count: 0.0}
+	return &StandardCounterFloat64{}
 }
 
 // NewCounterFloat64Forced constructs a new StandardCounterFloat64 and returns it no matter if
 // the global switch is enabled or not.
 func NewCounterFloat64Forced() CounterFloat64 {
-	return &StandardCounterFloat64{count: 0.0}
+	return &StandardCounterFloat64{}
 }
 
 // NewRegisteredCounterFloat64 constructs and registers a new StandardCounterFloat64.
@@ -113,41 +114,42 @@ func (NilCounterFloat64) Inc(i float64) {}
 func (NilCounterFloat64) Snapshot() CounterFloat64 { return NilCounterFloat64{} }
 
 // StandardCounterFloat64 is the standard implementation of a CounterFloat64 and uses the
-// sync.Mutex package to manage a single float64 value.
+// atomic to manage a single float64 value.
 type StandardCounterFloat64 struct {
-	mutex sync.Mutex
-	count float64
+	floatBits atomic.Uint64
 }
 
 // Clear sets the counter to zero.
 func (c *StandardCounterFloat64) Clear() {
-	c.mutex.Lock()
-	defer c.mutex.Unlock()
-	c.count = 0.0
+	c.floatBits.Store(0)
 }
 
 // Count returns the current value.
 func (c *StandardCounterFloat64) Count() float64 {
-	c.mutex.Lock()
-	defer c.mutex.Unlock()
-	return c.count
+	return math.Float64frombits(c.floatBits.Load())
 }
 
 // Dec decrements the counter by the given amount.
 func (c *StandardCounterFloat64) Dec(v float64) {
-	c.mutex.Lock()
-	defer c.mutex.Unlock()
-	c.count -= v
+	atomicAddFloat(&c.floatBits, -v)
 }
 
 // Inc increments the counter by the given amount.
 func (c *StandardCounterFloat64) Inc(v float64) {
-	c.mutex.Lock()
-	defer c.mutex.Unlock()
-	c.count += v
+	atomicAddFloat(&c.floatBits, v)
 }
 
 // Snapshot returns a read-only copy of the counter.
 func (c *StandardCounterFloat64) Snapshot() CounterFloat64 {
 	return CounterFloat64Snapshot(c.Count())
 }
+
+func atomicAddFloat(fbits *atomic.Uint64, v float64) {
+	for {
+		loadedBits := fbits.Load()
+		newBits := math.Float64bits(math.Float64frombits(loadedBits) + v)
+		if fbits.CompareAndSwap(loadedBits, newBits) {
+			break
+		}
+	}
+}

metrics/counter_float_64_test.go

 package metrics
 
-import "testing"
+import (
+	"sync"
+	"testing"
+)
 
 func BenchmarkCounterFloat64(b *testing.B) {
 	c := NewCounterFloat64()
@@ -10,6 +13,25 @@ func BenchmarkCounterFloat64(b *testing.B) {
 	}
 }
 
+func BenchmarkCounterFloat64Parallel(b *testing.B) {
+	c := NewCounterFloat64()
+	b.ResetTimer()
+	var wg sync.WaitGroup
+	for i := 0; i < 10; i++ {
+		wg.Add(1)
+		go func() {
+			for i := 0; i < b.N; i++ {
+				c.Inc(1.0)
+			}
+			wg.Done()
+		}()
+	}
+	wg.Wait()
+	if have, want := c.Count(), 10.0*float64(b.N); have != want {
+		b.Fatalf("have %f want %f", have, want)
+	}
+}
+
 func TestCounterFloat64Clear(t *testing.T) {
 	c := NewCounterFloat64()
 	c.Inc(1.0)

metrics/gauge_float64.go

 package metrics
 
-import "sync"
+import (
+	"math"
+	"sync/atomic"
+)
 
 // GaugeFloat64s hold a float64 value that can be set arbitrarily.
 type GaugeFloat64 interface {
@@ -23,9 +26,7 @@ func NewGaugeFloat64() GaugeFloat64 {
 	if !Enabled {
 		return NilGaugeFloat64{}
 	}
-	return &StandardGaugeFloat64{
-		value: 0.0,
-	}
+	return &StandardGaugeFloat64{}
 }
 
 // NewRegisteredGaugeFloat64 constructs and registers a new StandardGaugeFloat64.
@@ -83,10 +84,9 @@ func (NilGaugeFloat64) Update(v float64) {}
 func (NilGaugeFloat64) Value() float64 { return 0.0 }
 
 // StandardGaugeFloat64 is the standard implementation of a GaugeFloat64 and uses
-// sync.Mutex to manage a single float64 value.
+// atomic to manage a single float64 value.
 type StandardGaugeFloat64 struct {
-	mutex sync.Mutex
-	value float64
+	floatBits atomic.Uint64
 }
 
 // Snapshot returns a read-only copy of the gauge.
@@ -96,16 +96,12 @@ func (g *StandardGaugeFloat64) Snapshot() GaugeFloat64 {
 
 // Update updates the gauge's value.
 func (g *StandardGaugeFloat64) Update(v float64) {
-	g.mutex.Lock()
-	defer g.mutex.Unlock()
-	g.value = v
+	g.floatBits.Store(math.Float64bits(v))
 }
 
 // Value returns the gauge's current value.
 func (g *StandardGaugeFloat64) Value() float64 {
-	g.mutex.Lock()
-	defer g.mutex.Unlock()
-	return g.value
+	return math.Float64frombits(g.floatBits.Load())
 }
 
 // FunctionalGaugeFloat64 returns value from given function

metrics/gauge_float64_test.go

 package metrics
 
-import "testing"
+import (
+	"sync"
+	"testing"
+)
 
 func BenchmarkGaugeFloat64(b *testing.B) {
 	g := NewGaugeFloat64()
@@ -10,6 +13,24 @@ func BenchmarkGaugeFloat64(b *testing.B) {
 	}
 }
 
+func BenchmarkGaugeFloat64Parallel(b *testing.B) {
+	c := NewGaugeFloat64()
+	var wg sync.WaitGroup
+	for i := 0; i < 10; i++ {
+		wg.Add(1)
+		go func() {
+			for i := 0; i < b.N; i++ {
+				c.Update(float64(i))
+			}
+			wg.Done()
+		}()
+	}
+	wg.Wait()
+	if have, want := c.Value(), float64(b.N-1); have != want {
+		b.Fatalf("have %f want %f", have, want)
+	}
+}
+
 func TestGaugeFloat64(t *testing.T) {
 	g := NewGaugeFloat64()
 	g.Update(47.0)