Improve metrics error msging

api/metrics/multi_gatherer.go

@@ -16,9 +16,9 @@ import (
 )
 
 var (
-	errDuplicatedPrefix = errors.New("duplicated prefix")
-
 	_ MultiGatherer = (*multiGatherer)(nil)
+
+	errReregisterGatherer = errors.New("attempt to register existing gatherer")
 )
 
 // MultiGatherer extends the Gatherer interface by allowing additional gatherers
@@ -77,8 +77,8 @@ func (g *multiGatherer) Register(namespace string, gatherer prometheus.Gatherer)
 	g.lock.Lock()
 	defer g.lock.Unlock()
 
-	if _, exists := g.gatherers[namespace]; exists {
-		return errDuplicatedPrefix
+	if existingGatherer, exists := g.gatherers[namespace]; exists {
+		return fmt.Errorf("%w for namespace %q failed; existing: %#v", errReregisterGatherer, namespace, existingGatherer)
 	}
 
 	g.gatherers[namespace] = gatherer

api/metrics/multi_gatherer_test.go

@@ -30,7 +30,7 @@ func TestMultiGathererDuplicatedPrefix(t *testing.T) {
 	require.NoError(g.Register("", og))
 
 	err := g.Register("", og)
-	require.ErrorIs(err, errDuplicatedPrefix)
+	require.ErrorIs(err, errReregisterGatherer)
 
 	require.NoError(g.Register("lol", og))
 }

api/metrics/optional_gatherer.go

@@ -4,19 +4,15 @@
 package metrics
 
 import (
-	"errors"
+	"fmt"
 	"sync"
 
 	"github.com/prometheus/client_golang/prometheus"
 
 	dto "github.com/prometheus/client_model/go"
 )
 
-var (
-	errDuplicatedRegister = errors.New("duplicated register")
-
-	_ OptionalGatherer = (*optionalGatherer)(nil)
-)
+var _ OptionalGatherer = (*optionalGatherer)(nil)
 
 // OptionalGatherer extends the Gatherer interface by allowing the optional
 // registration of a single gatherer. If no gatherer is registered, Gather will
@@ -54,7 +50,7 @@ func (g *optionalGatherer) Register(gatherer prometheus.Gatherer) error {
 	defer g.lock.Unlock()
 
 	if g.gatherer != nil {
-		return errDuplicatedRegister
+		return fmt.Errorf("%w; %#v", errReregisterGatherer, g.gatherer)
 	}
 	g.gatherer = gatherer
 	return nil

api/metrics/optional_gatherer_test.go

@@ -32,7 +32,7 @@ func TestOptionalGathererDuplicated(t *testing.T) {
 
 	require.NoError(g.Register(og))
 	err := g.Register(og)
-	require.ErrorIs(err, errDuplicatedRegister)
+	require.ErrorIs(err, errReregisterGatherer)
 }
 
 func TestOptionalGathererAddedError(t *testing.T) {