metrics: Simplify Group interface

Get rid of RegisterWithInit method. Instead, check inside Register method if
the registered collector implements CollectorWithInit.

Signed-off-by: Anna Kapuscinska <anna@isovalent.com>

pkg/metrics/group.go

@@ -9,37 +9,34 @@ import (
 	"github.com/prometheus/client_golang/prometheus"
 )
 
-// CollectorWithInit extends prometheus.Collector with methods for metrics
-// initialization and checking constraints.
-type CollectorWithInit interface {
-	prometheus.Collector
+// initializer contains methods for metrics initialization and checking
+// constraints.
+type initializer interface {
 	IsConstrained() bool
 	Init()
 	InitForDocs()
 }
 
-// Group extends prometheus.Registerer with methods for metrics
-// initialization and checking constraints. It also includes
-// prometheus.Collector, because MetricsGroups are intended to be registered in
-// a root prometheus.Registry.
-type Group interface {
+// CollectorWithInit extends prometheus.Collector with initializer.
+type CollectorWithInit interface {
 	prometheus.Collector
+	initializer
+}
+
+// Group extends prometheus.Registerer with CollectorWithInit.
+// It represents a sub-registry of the root prometheus.Registry.
+type Group interface {
 	prometheus.Registerer
-	RegisterWithInit(CollectorWithInit) error
-	MustRegisterWithInit(CollectorWithInit)
+	CollectorWithInit
 	ExtendInit(func())
 	ExtendInitForDocs(func())
-	IsConstrained() bool
-	Init()
-	InitForDocs()
 }
 
 // metricsGroup wraps prometheus.Registry and implements Group
 type metricsGroup struct {
 	registry *prometheus.Registry
-	// If constrained is true, group will only accept metrics with constrained
-	// cardinality - but this is checked only for metrics registered with
-	// (Must)RegisterWithInit.
+	// If constrained is true, group will accept collectors implementing
+	// initializer only if they are constrained.
 	constrained     bool
 	initFunc        func()
 	initForDocsFunc func()
@@ -66,65 +63,61 @@ func (r *metricsGroup) Collect(ch chan<- prometheus.Metric) {
 }
 
 // Register implements Group (prometheus.Registerer).
+//
+// It wraps the Register method of the underlying registry. Additionally, if
+// the collector implements initializer, it:
+//   - checks constraints - attempt to register an unconstrained collector in
+//     a constrained group results in an error
+//   - extends Init and InitForDocs methods with initialization of the
+//     registered collector
 func (r *metricsGroup) Register(c prometheus.Collector) error {
-	return r.registry.Register(c)
+	cc, hasInit := c.(initializer)
+	if hasInit {
+		// check constraints
+		if r.IsConstrained() && !cc.IsConstrained() {
+			return errors.New("can't register unconstrained metrics in a constrained group")
+		}
+	}
+	// register
+	err := r.registry.Register(c)
+	if err != nil {
+		return err
+	}
+	if hasInit {
+		// extend init
+		r.ExtendInit(cc.Init)
+		r.ExtendInitForDocs(cc.InitForDocs)
+	}
+	return nil
 }
 
 // MustRegister implements Group (prometheus.Registerer).
 func (r *metricsGroup) MustRegister(cs ...prometheus.Collector) {
-	r.registry.MustRegister(cs...)
+	for _, c := range cs {
+		if err := r.Register(c); err != nil {
+			panic(err)
+		}
+	}
 }
 
 // Unregister implements Group (prometheus.Registerer).
 func (r *metricsGroup) Unregister(c prometheus.Collector) bool {
 	return r.registry.Unregister(c)
 }
 
-// IsConstrained implements Group.
+// IsConstrained implements Group (initializer).
 func (r *metricsGroup) IsConstrained() bool {
 	return r.constrained
 }
 
-// RegisterWithInit implements Group. It wraps Register method and
-// additionally:
-//   - checks constraints - attempt to register an unconstrained collector in
-//     a constrained group results in an error
-//   - extends Init and InitForDocs methods with initialization of the
-//     registered collector
-func (r *metricsGroup) RegisterWithInit(c CollectorWithInit) error {
-	// check constraints
-	if r.IsConstrained() && !c.IsConstrained() {
-		return errors.New("can't register unconstrained metrics in a constrained group")
-	}
-
-	// register
-	err := r.Register(c)
-	if err != nil {
-		return err
-	}
-
-	// extend init
-	r.ExtendInit(c.Init)
-	r.ExtendInitForDocs(c.InitForDocs)
-	return nil
-}
-
-// MustRegisterWithInit implements Group.
-func (r *metricsGroup) MustRegisterWithInit(c CollectorWithInit) {
-	err := r.RegisterWithInit(c)
-	if err != nil {
-		panic(err)
-	}
-}
-
-// Init implements Group.
+// Init implements Group (initializer).
 func (r *metricsGroup) Init() {
 	if r.initFunc != nil {
 		r.initFunc()
 	}
 }
 
-// InitForDocs implements Group.
+// InitForDocs implements Group (initializer).
 func (r *metricsGroup) InitForDocs() {
 	if r.initForDocsFunc != nil {
 		r.initForDocsFunc()