metrics: hardcode lazy metric construction codepath

motivation is to remove the dependency on reflect.Call

Throughout the codebase, GetOrRegister() is mostly used with lazy
construction while Register() is strictly used with eagerly constructed
metric objects. So it makes sense to isolate the lazy construction to
GetOrRegister() and make caller decide on which function makes sense
for them.

metrics/counter.go

@@ -7,10 +7,7 @@ import (
 // GetOrRegisterCounter returns an existing Counter or constructs and registers
 // a new Counter.
 func GetOrRegisterCounter(name string, r Registry) *Counter {
-	if r == nil {
+	return GetOrRegister(name, NewCounter, r).(*Counter)
-		r = DefaultRegistry
-	}
-	return r.GetOrRegister(name, NewCounter).(*Counter)
 }
 
 // NewCounter constructs a new Counter.

metrics/counter_float64.go

@@ -8,10 +8,7 @@ import (
 // GetOrRegisterCounterFloat64 returns an existing *CounterFloat64 or constructs and registers
 // a new CounterFloat64.
 func GetOrRegisterCounterFloat64(name string, r Registry) *CounterFloat64 {
-	if nil == r {
+	return GetOrRegister(name, NewCounterFloat64, r).(*CounterFloat64)
-		r = DefaultRegistry
-	}
-	return r.GetOrRegister(name, NewCounterFloat64).(*CounterFloat64)
 }
 
 // NewCounterFloat64 constructs a new CounterFloat64.

metrics/gauge.go

@@ -11,10 +11,7 @@ func (g GaugeSnapshot) Value() int64 { return int64(g) }
 // GetOrRegisterGauge returns an existing Gauge or constructs and registers a
 // new Gauge.
 func GetOrRegisterGauge(name string, r Registry) *Gauge {
-	if r == nil {
+	return GetOrRegister(name, NewGauge, r).(*Gauge)
-		r = DefaultRegistry
-	}
-	return r.GetOrRegister(name, NewGauge).(*Gauge)
 }
 
 // NewGauge constructs a new Gauge.

metrics/gauge_float64.go

@@ -8,10 +8,7 @@ import (
 // GetOrRegisterGaugeFloat64 returns an existing GaugeFloat64 or constructs and registers a
 // new GaugeFloat64.
 func GetOrRegisterGaugeFloat64(name string, r Registry) *GaugeFloat64 {
-	if nil == r {
+	return GetOrRegister(name, NewGaugeFloat64, r).(*GaugeFloat64)
-		r = DefaultRegistry
-	}
-	return r.GetOrRegister(name, NewGaugeFloat64()).(*GaugeFloat64)
 }
 
 // GaugeFloat64Snapshot is a read-only copy of a GaugeFloat64.

metrics/gauge_info.go

@@ -16,10 +16,7 @@ func (val GaugeInfoValue) String() string {
 // GetOrRegisterGaugeInfo returns an existing GaugeInfo or constructs and registers a
 // new GaugeInfo.
 func GetOrRegisterGaugeInfo(name string, r Registry) *GaugeInfo {
-	if nil == r {
+	return GetOrRegister(name, NewGaugeInfo, r).(*GaugeInfo)
-		r = DefaultRegistry
-	}
-	return r.GetOrRegister(name, NewGaugeInfo()).(*GaugeInfo)
 }
 
 // NewGaugeInfo constructs a new GaugeInfo.

metrics/histogram.go

@@ -23,19 +23,13 @@ type Histogram interface {
 // GetOrRegisterHistogram returns an existing Histogram or constructs and
 // registers a new StandardHistogram.
 func GetOrRegisterHistogram(name string, r Registry, s Sample) Histogram {
-	if nil == r {
+	return GetOrRegister(name, func() Histogram { return NewHistogram(s) }, r).(Histogram)
-		r = DefaultRegistry
-	}
-	return r.GetOrRegister(name, func() Histogram { return NewHistogram(s) }).(Histogram)
 }
 
 // GetOrRegisterHistogramLazy returns an existing Histogram or constructs and
 // registers a new StandardHistogram.
 func GetOrRegisterHistogramLazy(name string, r Registry, s func() Sample) Histogram {
-	if nil == r {
+	return GetOrRegister(name, func() Histogram { return NewHistogram(s()) }, r).(Histogram)
-		r = DefaultRegistry
-	}
-	return r.GetOrRegister(name, func() Histogram { return NewHistogram(s()) }).(Histogram)
 }
 
 // NewHistogram constructs a new StandardHistogram from a Sample.

metrics/meter.go

@@ -12,10 +12,7 @@ import (
 // Be sure to unregister the meter from the registry once it is of no use to
 // allow for garbage collection.
 func GetOrRegisterMeter(name string, r Registry) *Meter {
-	if r == nil {
+	return GetOrRegister(name, NewMeter, r).(*Meter)
-		r = DefaultRegistry
-	}
-	return r.GetOrRegister(name, NewMeter).(*Meter)
 }
 
 // NewMeter constructs a new Meter and launches a goroutine.

metrics/registry.go

@@ -3,7 +3,6 @@ package metrics
 import (
 	"errors"
 	"fmt"
-	"reflect"
 	"sort"
 	"strings"
 	"sync"
@@ -30,10 +29,9 @@ type Registry interface {
 	// GetAll metrics in the Registry.
 	GetAll() map[string]map[string]interface{}
 
-	// GetOrRegister gets an existing metric or registers the given one.
+	// GetOrRegister returns an existing metric or registers the one returned
-	// The interface can be the metric to register if not found in registry,
+	// by the given constructor.
-	// or a function returning the metric for lazy instantiation.
+	GetOrRegister(string, func() interface{}) interface{}
-	GetOrRegister(string, interface{}) interface{}
 
 	// Register the given metric under the given name.
 	Register(string, interface{}) error
@@ -95,19 +93,13 @@ func (r *StandardRegistry) Get(name string) interface{} {
 // alternative to calling Get and Register on failure.
 // The interface can be the metric to register if not found in registry,
 // or a function returning the metric for lazy instantiation.
-func (r *StandardRegistry) GetOrRegister(name string, i interface{}) interface{} {
+func (r *StandardRegistry) GetOrRegister(name string, ctor func() interface{}) interface{} {
 	// fast path
 	cached, ok := r.metrics.Load(name)
 	if ok {
 		return cached
 	}
-	if v := reflect.ValueOf(i); v.Kind() == reflect.Func {
+	item, _, _ := r.loadOrRegister(name, ctor())
-		i = v.Call(nil)[0].Interface()
-	}
-	item, _, ok := r.loadOrRegister(name, i)
-	if !ok {
-		return i
-	}
 	return item
 }
 
@@ -120,9 +112,6 @@ func (r *StandardRegistry) Register(name string, i interface{}) error {
 		return fmt.Errorf("%w: %v", ErrDuplicateMetric, name)
 	}
 
-	if v := reflect.ValueOf(i); v.Kind() == reflect.Func {
-		i = v.Call(nil)[0].Interface()
-	}
 	_, loaded, _ := r.loadOrRegister(name, i)
 	if loaded {
 		return fmt.Errorf("%w: %v", ErrDuplicateMetric, name)
@@ -295,9 +284,9 @@ func (r *PrefixedRegistry) Get(name string) interface{} {
 // GetOrRegister gets an existing metric or registers the given one.
 // The interface can be the metric to register if not found in registry,
 // or a function returning the metric for lazy instantiation.
-func (r *PrefixedRegistry) GetOrRegister(name string, metric interface{}) interface{} {
+func (r *PrefixedRegistry) GetOrRegister(name string, ctor func() interface{}) interface{} {
 	realName := r.prefix + name
-	return r.underlying.GetOrRegister(realName, metric)
+	return r.underlying.GetOrRegister(realName, ctor)
 }
 
 // Register the given metric under the given name. The name will be prefixed.
@@ -338,8 +327,11 @@ func Get(name string) interface{} {
 
 // GetOrRegister gets an existing metric or creates and registers a new one. Threadsafe
 // alternative to calling Get and Register on failure.
-func GetOrRegister(name string, i interface{}) interface{} {
+func GetOrRegister[T any](name string, ctor func() T, r Registry) interface{} {
-	return DefaultRegistry.GetOrRegister(name, i)
+	if nil == r {
+		r = DefaultRegistry
+	}
+	return r.GetOrRegister(name, func() any { return ctor() })
 }
 
 // Register the given metric under the given name.  Returns a ErrDuplicateMetric

metrics/registry_test.go

@@ -30,7 +30,7 @@ func benchmarkRegistryGetOrRegisterParallel(b *testing.B, amount int) {
 		wg.Add(1)
 		go func() {
 			for i := 0; i < b.N; i++ {
-				r.GetOrRegister("foo", NewMeter)
+				GetOrRegister("foo", NewMeter, r)
 			}
 			wg.Done()
 		}()
@@ -98,8 +98,8 @@ func TestRegistryGetOrRegister(t *testing.T) {
 	r := NewRegistry()
 
 	// First metric wins with GetOrRegister
-	_ = r.GetOrRegister("foo", NewCounter())
+	_ = GetOrRegister("foo", NewCounter, r)
-	m := r.GetOrRegister("foo", NewGauge())
+	m := GetOrRegister("foo", NewGauge, r)
 	if _, ok := m.(*Counter); !ok {
 		t.Fatal(m)
 	}
@@ -123,8 +123,8 @@ func TestRegistryGetOrRegisterWithLazyInstantiation(t *testing.T) {
 	r := NewRegistry()
 
 	// First metric wins with GetOrRegister
-	_ = r.GetOrRegister("foo", NewCounter)
+	_ = GetOrRegister("foo", NewCounter, r)
-	m := r.GetOrRegister("foo", NewGauge)
+	m := GetOrRegister("foo", NewGauge, r)
 	if _, ok := m.(*Counter); !ok {
 		t.Fatal(m)
 	}
@@ -165,7 +165,7 @@ func TestPrefixedChildRegistryGetOrRegister(t *testing.T) {
 	r := NewRegistry()
 	pr := NewPrefixedChildRegistry(r, "prefix.")
 
-	_ = pr.GetOrRegister("foo", NewCounter())
+	_ = GetOrRegister("foo", NewCounter, pr)
 
 	i := 0
 	r.Each(func(name string, m interface{}) {
@@ -182,7 +182,7 @@ func TestPrefixedChildRegistryGetOrRegister(t *testing.T) {
 func TestPrefixedRegistryGetOrRegister(t *testing.T) {
 	r := NewPrefixedRegistry("prefix.")
 
-	_ = r.GetOrRegister("foo", NewCounter())
+	_ = GetOrRegister("foo", NewCounter, r)
 
 	i := 0
 	r.Each(func(name string, m interface{}) {

metrics/resetting_timer.go

@@ -8,10 +8,7 @@ import (
 // GetOrRegisterResettingTimer returns an existing ResettingTimer or constructs and registers a
 // new ResettingTimer.
 func GetOrRegisterResettingTimer(name string, r Registry) *ResettingTimer {
-	if nil == r {
+	return GetOrRegister(name, NewResettingTimer, r).(*ResettingTimer)
-		r = DefaultRegistry
-	}
-	return r.GetOrRegister(name, NewResettingTimer).(*ResettingTimer)
 }
 
 // NewRegisteredResettingTimer constructs and registers a new ResettingTimer.

metrics/runtimehistogram.go

@@ -8,11 +8,8 @@ import (
 )
 
 func getOrRegisterRuntimeHistogram(name string, scale float64, r Registry) *runtimeHistogram {
-	if r == nil {
-		r = DefaultRegistry
-	}
 	constructor := func() Histogram { return newRuntimeHistogram(scale) }
-	return r.GetOrRegister(name, constructor).(*runtimeHistogram)
+	return GetOrRegister(name, constructor, r).(*runtimeHistogram)
 }
 
 // runtimeHistogram wraps a runtime/metrics histogram.

metrics/timer.go

@@ -10,10 +10,7 @@ import (
 // Be sure to unregister the meter from the registry once it is of no use to
 // allow for garbage collection.
 func GetOrRegisterTimer(name string, r Registry) *Timer {
-	if nil == r {
+	return GetOrRegister(name, NewTimer, r).(*Timer)
-		r = DefaultRegistry
-	}
-	return r.GetOrRegister(name, NewTimer).(*Timer)
 }
 
 // NewCustomTimer constructs a new Timer from a Histogram and a Meter.