Act on DefaultRegisterer and DefaultDeliverer where possible

Both point to the DefaultRegistry anyway, but users can now replace it
with interfaces rather than only other Registry instances.

prometheus/histogram.go

@@ -51,11 +51,11 @@ type Histogram interface {
 // bucket of a histogram ("le" -> "less or equal").
 const bucketLabel = "le"
 
-var (
 // DefBuckets are the default Histogram buckets. The default buckets are
-	// tailored to broadly measure the response time (in seconds) of a
-	// network service. Most likely, however, you will be required to define
-	// buckets customized to your use case.
+// tailored to broadly measure the response time (in seconds) of a network
+// service. Most likely, however, you will be required to define buckets
+// customized to your use case.
+var (
 	DefBuckets = []float64{.005, .01, .025, .05, .1, .25, .5, 1, 2.5, 5, 10}
 
 	errBucketLabelNotAllowed = fmt.Errorf(

prometheus/http.go

@@ -50,19 +50,19 @@ func Handler() http.Handler {
 	return InstrumentHandler("prometheus", UninstrumentedHandler())
 }
 
-// UninstrumentedHandler returns an HTTP handler for the DefaultRegistry. The
+// UninstrumentedHandler returns an HTTP handler for the DefaultDeliverer. The
 // Handler uses the default HandlerOpts, i.e. report the first error as an HTTP
 // error, no error logging, and compression if requested by the client.
 //
-// If you want to create a Handler for the DefaultRegistry with different
-// HandlerOpts, create it with HandlerFor with the DefaultRegistry and your
+// If you want to create a Handler for the DefaultDeliverer with different
+// HandlerOpts, create it with HandlerFor with the DefaultDeliverer and your
 // desired HandlerOpts.
 //
 // Note that in future versions of this package, UninstrumentedHandler will be
 // replaced by Handler (which will then return an uninstrumented handler, see
 // there for details).
 func UninstrumentedHandler() http.Handler {
-	return HandlerFor(DefaultRegistry, HandlerOpts{})
+	return HandlerFor(DefaultDeliverer, HandlerOpts{})
 }
 
 // HandlerFor returns an http.Handler for the provided Deliverer. The behavior ef

prometheus/registry.go

@@ -32,10 +32,19 @@ const (
 	capDescChan   = 10
 )
 
-// DefaultRegistry is the default registry implicitly used by a number of
-// convenience functions. It has a ProcessCollector and a GoCollector
-// pre-registered.
-var DefaultRegistry = NewRegistry()
+// DefaultRegistry is a Registry instance that has a ProcessCollector and a
+// GoCollector pre-registered. DefaultRegisterer and DefaultDeliverer are both
+// pointing to it. A number of convenience functions in this package act on
+// them. This approach to keep a default instance as global state mirrors the
+// approach of other packages in the Go standard library. Note that there are
+// caveats. Change the variables with caution and only if you understand the
+// consequences. Users who want to avoid global state altogether should not
+// use the convenience function and act on custom instances instead.
+var (
+	DefaultRegistry              = NewRegistry()
+	DefaultRegisterer Registerer = DefaultRegistry
+	DefaultDeliverer  Deliverer  = DefaultRegistry
+)
 
 func init() {
 	MustRegister(NewProcessCollector(os.Getpid(), ""))
@@ -70,9 +79,9 @@ func NewPedanticRegistry() *Registry {
 
 // Registerer is the interface for the part of a registry in charge of
 // registering and unregistering. Users of custom registries should use
-// Registerer as type for registration purposes (rather then Registry). In that
-// way, they are free to exchange the Registerer implementation (e.g. for
-// testing purposes).
+// Registerer as type for registration purposes (rather then the Registry type
+// directly). In that way, they are free to exchange the Registerer
+// implementation (e.g. for testing purposes).
 type Registerer interface {
 	// Register registers a new Collector to be included in metrics
 	// collection. It returns an error if the descriptors provided by the
@@ -107,7 +116,8 @@ type Registerer interface {
 }
 
 // Deliverer is the interface for the part of a registry in charge of delivering
-// the collected metrics.
+// the collected metrics, wich the same general implication as described for the
+// Registerer interface.
 type Deliverer interface {
 	// Deliver collects metrics from registered Collectors and returns them
 	// as lexicographically sorted MetricFamily protobufs. Even if an error
@@ -125,24 +135,24 @@ type Deliverer interface {
 	Deliver() ([]*dto.MetricFamily, error)
 }
 
-// Register registers the provided Collector with the DefaultRegistry.
+// Register registers the provided Collector with the DefaultRegisterer.
 //
-// Register is a shortcut for DefaultRegistry.Register(c). See there for more
+// Register is a shortcut for DefaultRegisterer.Register(c). See there for more
 // details.
 func Register(c Collector) error {
-	return DefaultRegistry.Register(c)
+	return DefaultRegisterer.Register(c)
 }
 
-// MustRegister registers the provided Collectors with the DefaultRegistry and
+// MustRegister registers the provided Collectors with the DefaultRegisterer and
 // panics if any error occurs.
 //
-// MustRegister is a shortcut for MustRegisterWith(DefaultRegistry, cs...). See
+// MustRegister is a shortcut for DefaultRegisterer.MustRegister(cs...). See
 // there for more details.
 func MustRegister(cs ...Collector) {
-	DefaultRegistry.MustRegister(cs...)
+	DefaultRegisterer.MustRegister(cs...)
 }
 
-// RegisterOrGet registers the provided Collector with the DefaultRegistry and
+// RegisterOrGet registers the provided Collector with the DefaultRegisterer and
 // returns the Collector, unless an equal Collector was registered before, in
 // which case that Collector is returned.
 //
@@ -175,12 +185,12 @@ func MustRegisterOrGet(c Collector) Collector {
 }
 
 // Unregister removes the registration of the provided Collector from the
-// DefaultRegistry.
+// DefaultRegisterer.
 //
-// Unregister is a shortcut for DefaultRegistry.Unregister(c). See there for
+// Unregister is a shortcut for DefaultRegisterer.Unregister(c). See there for
 // more details.
 func Unregister(c Collector) bool {
-	return DefaultRegistry.Unregister(c)
+	return DefaultRegisterer.Unregister(c)
 }
 
 // SetMetricFamilyInjectionHook sets a MetricFamily injection hook on the

prometheus/summary.go

@@ -53,8 +53,8 @@ type Summary interface {
 	Observe(float64)
 }
 
-var (
 // DefObjectives are the default Summary quantile values.
+var (
 	DefObjectives = map[float64]float64{0.5: 0.05, 0.9: 0.01, 0.99: 0.001}
 
 	errQuantileLabelNotAllowed = fmt.Errorf(