fix: tolerate legacy help text punctuation

prometheus/registry.go

@@ -679,7 +679,7 @@ func processMetric(
 	}
 	metricFamily, ok := metricFamiliesByName[desc.fqName]
 	if ok { // Existing name.
-		if metricFamily.GetHelp() != desc.help {
+		if !helpTextMatches(metricFamily.GetHelp(), desc.help) {
 			return fmt.Errorf(
 				"collected metric %s %s has help %q but should have %q",
 				desc.fqName, dtoMetric, desc.help, metricFamily.GetHelp(),
@@ -812,7 +812,7 @@ func (gs Gatherers) Gather() ([]*dto.MetricFamily, error) {
 		for _, mf := range mfs {
 			existingMF, exists := metricFamiliesByName[mf.GetName()]
 			if exists {
-				if existingMF.GetHelp() != mf.GetHelp() {
+				if !helpTextMatches(existingMF.GetHelp(), mf.GetHelp()) {
 					errs = append(errs, fmt.Errorf(
 						"gathered metric family %s has help %q but should have %q",
 						mf.GetName(), mf.GetHelp(), existingMF.GetHelp(),
@@ -1001,7 +1001,7 @@ func checkDescConsistency(
 	desc *Desc,
 ) error {
 	// Desc help consistency with metric family help.
-	if metricFamily.GetHelp() != desc.help {
+	if !helpTextMatches(metricFamily.GetHelp(), desc.help) {
 		return fmt.Errorf(
 			"collected metric %s %s has help %q but should have %q",
 			metricFamily.GetName(), dtoMetric, metricFamily.GetHelp(), desc.help,
@@ -1036,6 +1036,20 @@ func checkDescConsistency(
 	return nil
 }
 
+// helpTextMatches preserves compatibility with collectors that changed only a
+// final period in their help text. This is needed when applications using
+// different client versions share a registry, but other help-text changes
+// remain strict consistency errors.
+func helpTextMatches(a, b string) bool {
+	if a == b {
+		return true
+	}
+	if len(a) > 1 && strings.HasSuffix(a, ".") && strings.TrimSuffix(a, ".") == b {
+		return true
+	}
+	return len(b) > 1 && strings.HasSuffix(b, ".") && strings.TrimSuffix(b, ".") == a
+}
+
 var _ TransactionalGatherer = &MultiTRegistry{}
 
 // MultiTRegistry is a TransactionalGatherer that joins gathered metrics from multiple

prometheus/registry_test.go

@@ -748,6 +748,42 @@ func TestHandler(t *testing.T) {
 	testHandler(t)
 }
 
+func TestGatherAllowsHelpTextFinalPeriodDifference(t *testing.T) {
+	registry := prometheus.NewPedanticRegistry()
+	registry.MustRegister(prometheus.NewGauge(prometheus.GaugeOpts{
+		Name:        "period_compatibility",
+		Help:        "help text",
+		ConstLabels: prometheus.Labels{"source": "old"},
+	}))
+	registry.MustRegister(uncheckedCollector{c: prometheus.NewGauge(prometheus.GaugeOpts{
+		Name:        "period_compatibility",
+		Help:        "help text.",
+		ConstLabels: prometheus.Labels{"source": "new"},
+	})})
+
+	if _, err := registry.Gather(); err != nil {
+		t.Fatalf("Gather returned an error for a final-period-only difference: %v", err)
+	}
+}
+
+func TestGatherRejectsOtherHelpTextDifference(t *testing.T) {
+	registry := prometheus.NewPedanticRegistry()
+	registry.MustRegister(prometheus.NewGauge(prometheus.GaugeOpts{
+		Name:        "period_compatibility_strict",
+		Help:        "help text",
+		ConstLabels: prometheus.Labels{"source": "old"},
+	}))
+	registry.MustRegister(uncheckedCollector{c: prometheus.NewGauge(prometheus.GaugeOpts{
+		Name:        "period_compatibility_strict",
+		Help:        "different help text",
+		ConstLabels: prometheus.Labels{"source": "new"},
+	})})
+
+	if _, err := registry.Gather(); err == nil {
+		t.Fatal("Gather accepted a non-period help-text difference")
+	}
+}
+
 func BenchmarkHandler(b *testing.B) {
 	for i := 0; i < b.N; i++ {
 		testHandler(b)