add otel sdk compatibility test

Signed-off-by: Jay DeLuca <jaydeluca4@gmail.com>

Author: jaydeluca

prometheus-metrics-model/src/test/java/io/prometheus/metrics/model/registry/OpenTelemetryExporterRegistryCompatibilityTest.java

@@ -0,0 +1,116 @@
+package io.prometheus.metrics.model.registry;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatCode;
+
+import io.prometheus.metrics.model.snapshots.CounterSnapshot;
+import io.prometheus.metrics.model.snapshots.GaugeSnapshot;
+import io.prometheus.metrics.model.snapshots.MetricSnapshot;
+import io.prometheus.metrics.model.snapshots.MetricSnapshots;
+import java.util.Collections;
+import org.junit.jupiter.api.Test;
+
+/**
+ * Tests that use the Prometheus registry in the same way as the OpenTelemetry Java SDK Prometheus
+ * exporter ({@code io.opentelemetry.exporter.prometheus}). The SDK's {@code PrometheusMetricReader}
+ * implements {@link MultiCollector} with default implementations for all optional methods: {@link
+ * MultiCollector#getPrometheusNames()} returns an empty list, and {@link
+ * MultiCollector#getMetricType(String)}, {@link MultiCollector#getLabelNames(String)}, and {@link
+ * MultiCollector#getMetadata(String)} return null. This test suite ensures that registration,
+ * scrape, and unregister continue to work for that usage pattern and that a shared registry with
+ * both SDK-style and validated collectors behaves correctly.
+ */
+class OpenTelemetryExporterRegistryCompatibilityTest {
+
+  /**
+   * A MultiCollector that mimics the OpenTelemetry Java SDK's PrometheusMetricReader: it does not
+   * override getPrometheusNames() (empty list), getMetricType(String), getLabelNames(String), or
+   * getMetadata(String) (all null). Only collect() is implemented and returns MetricSnapshots.
+   */
+  private static final MultiCollector OTEL_STYLE_MULTI_COLLECTOR =
+      new MultiCollector() {
+        @Override
+        public MetricSnapshots collect() {
+          return new MetricSnapshots(
+              CounterSnapshot.builder()
+                  .name("otel_metric")
+                  .help("A metric produced by an OTel-style converter")
+                  .dataPoint(CounterSnapshot.CounterDataPointSnapshot.builder().value(42.0).build())
+                  .build());
+        }
+      };
+
+  @Test
+  void registerOtelStyleMultiCollector_succeeds() {
+    PrometheusRegistry registry = new PrometheusRegistry();
+
+    assertThatCode(() -> registry.register(OTEL_STYLE_MULTI_COLLECTOR)).doesNotThrowAnyException();
+  }
+
+  @Test
+  void scrape_afterRegisteringOtelStyleMultiCollector_returnsSnapshotsFromCollector() {
+    PrometheusRegistry registry = new PrometheusRegistry();
+    registry.register(OTEL_STYLE_MULTI_COLLECTOR);
+
+    MetricSnapshots snapshots = registry.scrape();
+
+    assertThat(snapshots).hasSize(1);
+    MetricSnapshot snapshot = snapshots.get(0);
+    assertThat(snapshot.getMetadata().getPrometheusName()).isEqualTo("otel_metric");
+  }
+
+  @Test
+  void unregisterOtelStyleMultiCollector_succeedsAndScrapeNoLongerIncludesIt() {
+    PrometheusRegistry registry = new PrometheusRegistry();
+    registry.register(OTEL_STYLE_MULTI_COLLECTOR);
+
+    assertThat(registry.scrape()).hasSize(1);
+
+    assertThatCode(() -> registry.unregister(OTEL_STYLE_MULTI_COLLECTOR))
+        .doesNotThrowAnyException();
+
+    assertThat(registry.scrape()).isEmpty();
+  }
+
+  @Test
+  void sharedRegistry_otelStyleMultiCollectorAndValidatedCollector_bothParticipateInScrape() {
+    PrometheusRegistry registry = new PrometheusRegistry();
+
+    Collector validatedCollector =
+        new Collector() {
+          @Override
+          public MetricSnapshot collect() {
+            return GaugeSnapshot.builder().name("app_gauge").help("App gauge").build();
+          }
+
+          @Override
+          public String getPrometheusName() {
+            return "app_gauge";
+          }
+
+          @Override
+          public MetricType getMetricType() {
+            return MetricType.GAUGE;
+          }
+
+          @Override
+          public java.util.Set<String> getLabelNames() {
+            return Collections.emptySet();
+          }
+        };
+
+    registry.register(validatedCollector);
+    registry.register(OTEL_STYLE_MULTI_COLLECTOR);
+
+    MetricSnapshots snapshots = registry.scrape();
+
+    assertThat(snapshots).hasSize(2);
+    assertThat(snapshots)
+        .extracting(s -> s.getMetadata().getPrometheusName())
+        .containsExactlyInAnyOrder("app_gauge", "otel_metric");
+
+    registry.unregister(OTEL_STYLE_MULTI_COLLECTOR);
+    assertThat(registry.scrape()).hasSize(1);
+    assertThat(registry.scrape().get(0).getMetadata().getPrometheusName()).isEqualTo("app_gauge");
+  }
+}