fix: omit OpenMetrics histogram sums for negative buckets

prometheus-metrics-exposition-textformats/src/main/java/io/prometheus/metrics/expositionformats/OpenMetricsTextFormatWriter.java

@@ -179,11 +179,11 @@ void writeHistogram(Writer writer, HistogramSnapshot snapshot, EscapingScheme sc
     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 @@ private void writeClassicHistogramBuckets(
       MetricMetadata metadata,
       String countSuffix,
       String sumSuffix,
+      boolean gaugeHistogram,
       List<HistogramSnapshot.HistogramDataPointSnapshot> dataList,
       EscapingScheme scheme)
       throws IOException {
@@ -225,7 +226,9 @@ private void writeClassicHistogramBuckets(
         writeScrapeTimestampAndExemplar(writer, data, exemplar, scheme);
       }
       // In OpenMetrics format, histogram _count and _sum are either both present or both absent.
-      if (data.hasCount() && data.hasSum()) {
+      // Negative threshold buckets preclude a sum for histograms, but not gauge histograms.
+      boolean hasNegativeBuckets = buckets.getUpperBound(0) < 0;
+      if (data.hasCount() && data.hasSum() && (gaugeHistogram || !hasNegativeBuckets)) {
         writeCountAndSum(writer, countName, sumName, data, exemplars, scheme, nameValidLegacy);
       }
       writeCreated(writer, name, data, scheme, nameValidLegacy);

prometheus-metrics-exposition-textformats/src/test/java/io/prometheus/metrics/expositionformats/NegativeBucketHistogramTest.java

@@ -0,0 +1,125 @@
+package io.prometheus.metrics.expositionformats;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import io.prometheus.metrics.model.snapshots.ClassicHistogramBuckets;
+import io.prometheus.metrics.model.snapshots.HistogramSnapshot;
+import io.prometheus.metrics.model.snapshots.MetricSnapshots;
+import java.io.ByteArrayOutputStream;
+import java.io.IOException;
+import java.nio.charset.StandardCharsets;
+import org.junit.jupiter.api.Test;
+
+class NegativeBucketHistogramTest {
+
+  @Test
+  void histogramWithNegativeBucketThresholdOmitsCountAndSum() throws IOException {
+    HistogramSnapshot histogram =
+        HistogramSnapshot.builder()
+            .name("my_histogram")
+            .dataPoint(
+                HistogramSnapshot.HistogramDataPointSnapshot.builder()
+                    .sum(0.5)
+                    .classicHistogramBuckets(
+                        ClassicHistogramBuckets.builder()
+                            .bucket(-1.0, 0)
+                            .bucket(1.0, 1)
+                            .bucket(Double.POSITIVE_INFINITY, 0)
+                            .build())
+                    .build())
+            .build();
+
+    assertThat(write(histogram))
+        .isEqualTo(
+            "# TYPE my_histogram histogram\n"
+                + "my_histogram_bucket{le=\"-1.0\"} 0\n"
+                + "my_histogram_bucket{le=\"1.0\"} 1\n"
+                + "my_histogram_bucket{le=\"+Inf\"} 1\n"
+                + "# EOF\n");
+  }
+
+  @Test
+  void histogramWithNonnegativeBucketThresholdKeepsCountAndSum() throws IOException {
+    HistogramSnapshot histogram =
+        HistogramSnapshot.builder()
+            .name("my_histogram")
+            .dataPoint(
+                HistogramSnapshot.HistogramDataPointSnapshot.builder()
+                    .sum(0.0)
+                    .classicHistogramBuckets(
+                        ClassicHistogramBuckets.builder()
+                            .bucket(0.0, 1)
+                            .bucket(Double.POSITIVE_INFINITY, 0)
+                            .build())
+                    .build())
+            .build();
+
+    assertThat(write(histogram))
+        .isEqualTo(
+            "# TYPE my_histogram histogram\n"
+                + "my_histogram_bucket{le=\"0.0\"} 1\n"
+                + "my_histogram_bucket{le=\"+Inf\"} 1\n"
+                + "my_histogram_count 1\n"
+                + "my_histogram_sum 0.0\n"
+                + "# EOF\n");
+  }
+
+  @Test
+  void histogramWithPositiveBucketThresholdKeepsCountAndSum() throws IOException {
+    HistogramSnapshot histogram =
+        HistogramSnapshot.builder()
+            .name("my_histogram")
+            .dataPoint(
+                HistogramSnapshot.HistogramDataPointSnapshot.builder()
+                    .sum(0.5)
+                    .classicHistogramBuckets(
+                        ClassicHistogramBuckets.builder()
+                            .bucket(1.0, 1)
+                            .bucket(Double.POSITIVE_INFINITY, 0)
+                            .build())
+                    .build())
+            .build();
+
+    assertThat(write(histogram))
+        .isEqualTo(
+            "# TYPE my_histogram histogram\n"
+                + "my_histogram_bucket{le=\"1.0\"} 1\n"
+                + "my_histogram_bucket{le=\"+Inf\"} 1\n"
+                + "my_histogram_count 1\n"
+                + "my_histogram_sum 0.5\n"
+                + "# EOF\n");
+  }
+
+  @Test
+  void gaugeHistogramWithNegativeBucketThresholdKeepsNegativeSum() throws IOException {
+    HistogramSnapshot histogram =
+        HistogramSnapshot.builder()
+            .name("my_gauge_histogram")
+            .gaugeHistogram(true)
+            .dataPoint(
+                HistogramSnapshot.HistogramDataPointSnapshot.builder()
+                    .sum(-7.0)
+                    .classicHistogramBuckets(
+                        ClassicHistogramBuckets.builder()
+                            .bucket(-1.0, 4)
+                            .bucket(Double.POSITIVE_INFINITY, 1)
+                            .build())
+                    .build())
+            .build();
+
+    assertThat(write(histogram))
+        .isEqualTo(
+            "# TYPE my_gauge_histogram gaugehistogram\n"
+                + "my_gauge_histogram_bucket{le=\"-1.0\"} 4\n"
+                + "my_gauge_histogram_bucket{le=\"+Inf\"} 5\n"
+                + "my_gauge_histogram_gcount 5\n"
+                + "my_gauge_histogram_gsum -7.0\n"
+                + "# EOF\n");
+  }
+
+  private static String write(HistogramSnapshot histogram) throws IOException {
+    ByteArrayOutputStream out = new ByteArrayOutputStream();
+    OpenMetricsTextFormatWriter.create().write(out, MetricSnapshots.of(histogram));
+    return out.toString(StandardCharsets.UTF_8);
+  }
+}