Pull out ...WithExemplar methods into separate interfaces

This is, sadly, the only way to avoid a breaking change. The cost is
that anyone using exemplars has to perform a type assertion. This is,
however, a common pattern where interfaces turn out to need additional
methods in a stable library or only some implementations can provide
the additional methods (AKA "interface upgrade").

Needless to say that in v2 of this library, we'll do things in a more
straight forward way.

Signed-off-by: beorn7 <beorn@grafana.com>

examples/random/main.go

@@ -90,9 +90,10 @@ func main() {
 		for {
 			v := (rand.NormFloat64() * *normDomain) + *normMean
 			rpcDurations.WithLabelValues("normal").Observe(v)
-			rpcDurationsHistogram.ObserveWithExemplar(
-				// Demonstrate exemplar support with a dummy ID. This would be
-				// something like a trace ID in a real application.
+			// Demonstrate exemplar support with a dummy ID. This would be
+			// something like a trace ID in a real application.
+			// Note the necessary type assertion.
+			rpcDurationsHistogram.(prometheus.ExemplarObserver).ObserveWithExemplar(
 				v, prometheus.Labels{"dummyID": fmt.Sprint(rand.Intn(100000))},
 			)
 			time.Sleep(time.Duration(75*oscillationFactor()) * time.Millisecond)

prometheus/counter.go

@@ -41,13 +41,18 @@ type Counter interface {
 	// Add adds the given value to the counter. It panics if the value is <
 	// 0.
 	Add(float64)
-	// AddWithExemplar works like Add but also replaces the currently saved
-	// exemplar (if any) with a new one, created from the provided value,
-	// the current time as timestamp, and the provided labels. Empty Labels
-	// will lead to a valid (label-less) exemplar. But if Labels is nil, the
-	// current exemplar is left in place. This method panics if the value is
-	// < 0, if any of the provided labels are invalid, or if the provided
-	// labels contain more than 64 runes in total.
+}
+
+// ExemplarAdder is implemented by Counters that offer the option of adding a
+// value to the Counter together with an exemplar. Its AddWithExemplar method
+// works like the Add method of the Counter interface but also replaces the
+// currently saved exemplar (if any) with a new one, created from the provided
+// value, the current time as timestamp, and the provided labels. Empty Labels
+// will lead to a valid (label-less) exemplar. But if Labels is nil, the current
+// exemplar is left in place. AddWithExemplar panics if the value is < 0, if any
+// of the provided labels are invalid, or if the provided labels contain more
+// than 64 runes in total.
+type ExemplarAdder interface {
 	AddWithExemplar(value float64, exemplar Labels)
 }
 
@@ -56,6 +61,9 @@ type CounterOpts Opts
 
 // NewCounter creates a new Counter based on the provided CounterOpts.
 //
+// The returned implementation also implements ExemplarAdder. It is safe to
+// perform the corresponding type assertion.
+//
 // The returned implementation tracks the counter value in two separate
 // variables, a float64 and a uint64. The latter is used to track calls of the
 // Inc method and calls of the Add method with a value that can be represented

prometheus/histogram.go

@@ -48,15 +48,6 @@ type Histogram interface {
 
 	// Observe adds a single observation to the histogram.
 	Observe(float64)
-	// ObserveWithExemplar works like Observe but also replaces the
-	// currently saved exemplar for the relevant bucket (possibly none) with
-	// a new one, created from the provided value, the current time as
-	// timestamp, and the provided Labels. Empty Labels will lead to a valid
-	// (label-less) exemplar. But if Labels is nil, the current exemplar in
-	// the relevant bucket is left in place. This method panics if any of
-	// the provided labels are invalid or if the provided labels contain
-	// more than 64 runes in total.
-	ObserveWithExemplar(value float64, exemplar Labels)
 }
 
 // bucketLabel is used for the label that defines the upper bound of a
@@ -161,6 +152,10 @@ type HistogramOpts struct {
 
 // NewHistogram creates a new Histogram based on the provided HistogramOpts. It
 // panics if the buckets in HistogramOpts are not in strictly increasing order.
+//
+// The returned implementation also implements ExemplarObserver. It is safe to
+// perform the corresponding type assertion. Exemplars are tracked separately
+// for each bucket.
 func NewHistogram(opts HistogramOpts) Histogram {
 	return newHistogram(
 		NewDesc(

prometheus/observer.go

@@ -50,3 +50,15 @@ type ObserverVec interface {
 
 	Collector
 }
+
+// ExemplarObserver is implemented by Observers that offer the option of
+// observing a value together with an exemplar. Its ObserveWithExemplar method
+// works like the Observe method of an Observer but also replaces the currently
+// saved exemplar (if any) with a new one, created from the provided value, the
+// current time as timestamp, and the provided Labels. Empty Labels will lead to
+// a valid (label-less) exemplar. But if Labels is nil, the current exemplar is
+// left in place. ObserveWithExemplar panics if any of the provided labels are
+// invalid or if the provided labels contain more than 64 runes in total.
+type ExemplarObserver interface {
+	ObserveWithExemplar(value float64, exemplar Labels)
+}