Add logic for handling content negotation flag with OM2

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

Author: jaydeluca

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

@@ -63,9 +63,17 @@ public ExpositionFormatWriter findWriter(@Nullable String acceptHeader) {
       return prometheusProtobufWriter;
     }
 
-    // Prefer OM2 over OM1 when any OM2 feature is enabled
     if (isOpenMetrics2Enabled() && openMetrics2TextFormatWriter.accepts(acceptHeader)) {
-      return openMetrics2TextFormatWriter;
+      if (openMetrics2TextFormatWriter.getOpenMetrics2Properties().getContentNegotiation()) {
+        String version = parseOpenMetricsVersion(acceptHeader);
+        if ("2.0.0".equals(version)) {
+          return openMetrics2TextFormatWriter;
+        }
+        // version=1.0.0 or no version: fall through to OM1
+      } else {
+        // contentNegotiation=false: OM2 handles all OpenMetrics requests
+        return openMetrics2TextFormatWriter;
+      }
     }
 
     if (openMetricsTextFormatWriter.accepts(acceptHeader)) {
@@ -94,4 +102,22 @@ public OpenMetricsTextFormatWriter getOpenMetricsTextFormatWriter() {
   public OpenMetrics2TextFormatWriter getOpenMetrics2TextFormatWriter() {
     return openMetrics2TextFormatWriter;
   }
+
+  @Nullable
+  private static String parseOpenMetricsVersion(@Nullable String acceptHeader) {
+    if (acceptHeader == null) {
+      return null;
+    }
+    for (String mediaType : acceptHeader.split(";")) {
+      String[] tokens = mediaType.split("=");
+      if (tokens.length == 2) {
+        String key = tokens[0].trim();
+        String value = tokens[1].trim();
+        if (key.equals("version")) {
+          return value;
+        }
+      }
+    }
+    return null;
+  }
 }

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

@@ -155,7 +155,8 @@ void testOM2EnabledWithFeatureFlags() {
                     .build())
             .build();
     ExpositionFormats formats = ExpositionFormats.init(props);
-    ExpositionFormatWriter writer = formats.findWriter("application/openmetrics-text");
+    ExpositionFormatWriter writer =
+        formats.findWriter("application/openmetrics-text; version=2.0.0");
     assertThat(writer).isInstanceOf(OpenMetrics2TextFormatWriter.class);
   }
 
@@ -173,6 +174,63 @@ void testProtobufWriterTakesPrecedence() {
     assertThat(writer).isInstanceOf(PrometheusProtobufWriter.class);
   }
 
+  @Test
+  void testOM2ContentNegotiationWithVersion2() {
+    PrometheusProperties props =
+        PrometheusProperties.builder()
+            .openMetrics2Properties(
+                OpenMetrics2Properties.builder().enabled(true).contentNegotiation(true).build())
+            .build();
+    ExpositionFormats formats = ExpositionFormats.init(props);
+    ExpositionFormatWriter writer =
+        formats.findWriter("application/openmetrics-text; version=2.0.0");
+    assertThat(writer).isInstanceOf(OpenMetrics2TextFormatWriter.class);
+  }
+
+  @Test
+  void testOM2ContentNegotiationWithVersion1() {
+    PrometheusProperties props =
+        PrometheusProperties.builder()
+            .openMetrics2Properties(
+                OpenMetrics2Properties.builder().enabled(true).contentNegotiation(true).build())
+            .build();
+    ExpositionFormats formats = ExpositionFormats.init(props);
+    ExpositionFormatWriter writer =
+        formats.findWriter("application/openmetrics-text; version=1.0.0");
+    assertThat(writer).isInstanceOf(OpenMetricsTextFormatWriter.class);
+  }
+
+  @Test
+  void testOM2ContentNegotiationWithNoVersion() {
+    // When contentNegotiation=true and Accept header has no version, use OM1 writer
+    PrometheusProperties props =
+        PrometheusProperties.builder()
+            .openMetrics2Properties(
+                OpenMetrics2Properties.builder().enabled(true).contentNegotiation(true).build())
+            .build();
+    ExpositionFormats formats = ExpositionFormats.init(props);
+    ExpositionFormatWriter writer = formats.findWriter("application/openmetrics-text");
+    assertThat(writer).isInstanceOf(OpenMetricsTextFormatWriter.class);
+  }
+
+  @Test
+  void testOM2ContentNegotiationDisabled() {
+    PrometheusProperties props =
+        PrometheusProperties.builder()
+            .openMetrics2Properties(
+                OpenMetrics2Properties.builder().enabled(true).contentNegotiation(false).build())
+            .build();
+    ExpositionFormats formats = ExpositionFormats.init(props);
+    ExpositionFormatWriter writer1 = formats.findWriter("application/openmetrics-text");
+    ExpositionFormatWriter writer2 =
+        formats.findWriter("application/openmetrics-text; version=1.0.0");
+    ExpositionFormatWriter writer3 =
+        formats.findWriter("application/openmetrics-text; version=2.0.0");
+    assertThat(writer1).isInstanceOf(OpenMetrics2TextFormatWriter.class);
+    assertThat(writer2).isInstanceOf(OpenMetrics2TextFormatWriter.class);
+    assertThat(writer3).isInstanceOf(OpenMetrics2TextFormatWriter.class);
+  }
+
   @Test
   void testCounterComplete() throws IOException {
     String openMetricsText =