OpenMetrics writer exposes _count/_sum for classic histograms with negative bucket thresholds #2631

--- a/prometheus_client/metrics.py
+++ b/prometheus_client/metrics.py
@@ -566,7 +566,8 @@ def _child_samples(self):
             acc += self._buckets[i].get()
             samples.append(('_bucket', {'le': floatToGoString(bound)}, acc))
         samples.append(('_count', {}, acc))
-        samples.append(('_sum', {}, self._sum.get()))
+        if self._upper_bounds[0] >= 0:
+            samples.append(('_sum', {}, self._sum.get()))
         samples.append(('_created', {}, self._created))
         return tuple(samples)
 

--- a/prometheus_client/metrics_core.py
+++ b/prometheus_client/metrics_core.py
@@ -217,10 +217,13 @@ def add_metric(self, labels, buckets, sum_value, timestamp=None):
                 exemplar,
             ))
         # +Inf is last and provides the count value.
-        self.samples.extend([
-            Sample(self.name + '_count', dict(zip(self._labelnames, labels)), buckets[-1][1], timestamp),
-            Sample(self.name + '_sum', dict(zip(self._labelnames, labels)), sum_value, timestamp),
-        ])
+        self.samples.append(
+                Sample(self.name + '_count', dict(zip(self._labelnames, labels)), buckets[-1][1], timestamp))
+        # Don't iunclude sum if there's negative buckets.
+        if float(buckets[0][0]) >= 0 and sum_value is not None:
+            self.samples.append(
+                    Sample(self.name + '_sum', dict(zip(self._labelnames, labels)), sum_value, timestamp))
+

diff --git a/prometheus-metrics-exposition-textformats/src/main/java/io/prometheus/metrics/expositionformats/OpenMetricsTextFormatWriter.java b/prometheus-metrics-exposition-textformats/src/main/java/io/prometheus/metrics/expositionformats/OpenMetricsTextFormatWriter.java
--- a/prometheus-metrics-exposition-textformats/src/main/java/io/prometheus/metrics/expositionformats/OpenMetricsTextFormatWriter.java
+++ b/prometheus-metrics-exposition-textformats/src/main/java/io/prometheus/metrics/expositionformats/OpenMetricsTextFormatWriter.java
@@ -179,11 +179,11 @@ public class OpenMetricsTextFormatWriter implements ExpositionFormatWriter {
     if (snapshot.isGaugeHistogram()) {
       writeMetadata(writer, "gaugehistogram", metadata, scheme);
       writeClassicHistogramBuckets(
-          writer, metadata, "_gcount", "_gsum", snapshot.getDataPoints(), scheme);
+          writer, metadata, "_gcount", "_gsum", true, snapshot.getDataPoints(), scheme);
     } else {
       writeMetadata(writer, "histogram", metadata, scheme);
       writeClassicHistogramBuckets(
-          writer, metadata, "_count", "_sum", snapshot.getDataPoints(), scheme);
+          writer, metadata, "_count", "_sum", false, snapshot.getDataPoints(), scheme);
     }
   }
 
@@ -192,6 +192,7 @@ public class OpenMetricsTextFormatWriter implements ExpositionFormatWriter {
       MetricMetadata metadata,
       String countSuffix,
       String sumSuffix,
+      boolean gaugeHistogram,
       List<HistogramSnapshot.HistogramDataPointSnapshot> dataList,
       EscapingScheme scheme)
       throws IOException {
@@ -225,7 +226,9 @@ public class OpenMetricsTextFormatWriter implements ExpositionFormatWriter {
         writeScrapeTimestampAndExemplar(writer, data, exemplar, scheme);
       }
       // In OpenMetrics format, histogram _count and _sum are either both present or both absent.
-      if (data.hasCount() && data.hasSum()) {
+      // A histogram with negative buckets must not have a sum (a gauge histogram may).
+      boolean negativeBuckets = buckets.getUpperBound(0) < 0;
+      if (data.hasCount() && data.hasSum() && (gaugeHistogram || !negativeBuckets)) {
         writeCountAndSum(writer, countName, sumName, data, exemplars, scheme, nameValidLegacy);
       }
       writeCreated(writer, name, data, scheme, nameValidLegacy);