[improve][monitor] Add version=0.0.4 to /metrics content type for Prometheus 3.x compatibility (apache#24060)

(cherry picked from commit 3f45154)
(cherry picked from commit 99dbc3b)

Author: lhotari

pulsar-broker-common/src/main/java/org/apache/pulsar/broker/stats/prometheus/PrometheusMetricsServlet.java

@@ -40,6 +40,11 @@
 
 public class PrometheusMetricsServlet extends HttpServlet {
     public static final String DEFAULT_METRICS_PATH = "/metrics";
+    // Prometheus uses version 0.0.4 of the text format for metrics.
+    // This content-type value ensures compatibility with Prometheus 3.x and later versions.
+    // For details, refer to the Prometheus 3.x migration guide:
+    // https://prometheus.io/docs/prometheus/latest/migration/#scrape-protocols
+    public static final String PROMETHEUS_CONTENT_TYPE_004 = "text/plain; version=0.0.4; charset=utf-8";
     private static final long serialVersionUID = 1L;
     static final int HTTP_STATUS_OK_200 = 200;
     static final int HTTP_STATUS_INTERNAL_SERVER_ERROR_500 = 500;
@@ -164,7 +169,7 @@ private void handleAsyncMetricsRequest(AsyncContext context) {
 
     private void generateMetricsSynchronously(HttpServletResponse res) throws IOException {
         res.setStatus(HTTP_STATUS_OK_200);
-        res.setContentType("text/plain;charset=utf-8");
+        res.setContentType(PROMETHEUS_CONTENT_TYPE_004);
         PrometheusMetricsGeneratorUtils.generate(cluster, res.getOutputStream(), metricsProviders);
     }
 

pulsar-broker/src/main/java/org/apache/pulsar/broker/stats/prometheus/PulsarPrometheusMetricsServlet.java

@@ -145,7 +145,7 @@ public void onStartAsync(AsyncEvent event) throws IOException {
                     response.setStatus(HTTP_STATUS_INTERNAL_SERVER_ERROR_500);
                 } else {
                     response.setStatus(HTTP_STATUS_OK_200);
-                    response.setContentType("text/plain;charset=utf-8");
+                    response.setContentType(PROMETHEUS_CONTENT_TYPE_004);
                     if (compressOutput) {
                         response.setHeader("Content-Encoding", "gzip");
                     }

pulsar-broker/src/test/java/org/apache/pulsar/broker/service/BrokerServiceTest.java

@@ -88,6 +88,7 @@
 import org.apache.pulsar.broker.service.BrokerServiceException.PersistenceException;
 import org.apache.pulsar.broker.service.persistent.PersistentSubscription;
 import org.apache.pulsar.broker.service.persistent.PersistentTopic;
+import org.apache.pulsar.broker.stats.prometheus.PrometheusMetricsServlet;
 import org.apache.pulsar.broker.stats.prometheus.PrometheusRawMetricsProvider;
 import org.apache.pulsar.client.admin.BrokerStats;
 import org.apache.pulsar.client.admin.PulsarAdminException;
@@ -1472,6 +1473,8 @@ public void testMetricsProvider() throws IOException {
         HttpClient httpClient = HttpClientBuilder.create().build();
         final String metricsEndPoint = getPulsar().getWebServiceAddress() + "/metrics";
         HttpResponse response = httpClient.execute(new HttpGet(metricsEndPoint));
+        assertEquals(response.getEntity().getContentType().getValue(),
+                PrometheusMetricsServlet.PROMETHEUS_CONTENT_TYPE_004);
         InputStream inputStream = response.getEntity().getContent();
         InputStreamReader isReader = new InputStreamReader(inputStream);
         BufferedReader reader = new BufferedReader(isReader);

pulsar-functions/worker/src/main/java/org/apache/pulsar/functions/worker/rest/api/FunctionsMetricsResource.java

@@ -32,6 +32,7 @@
 import javax.ws.rs.core.MediaType;
 import javax.ws.rs.core.Response;
 import javax.ws.rs.core.StreamingOutput;
+import org.apache.pulsar.broker.stats.prometheus.PrometheusMetricsServlet;
 import org.apache.pulsar.common.util.SimpleTextOutputStream;
 import org.apache.pulsar.functions.worker.WorkerService;
 import org.apache.pulsar.functions.worker.rest.FunctionApiResource;
@@ -63,7 +64,7 @@ public Response getMetrics() throws IOException {
             };
             return Response
                 .ok(streamOut)
-                .type(MediaType.TEXT_PLAIN_TYPE)
+                .type(PrometheusMetricsServlet.PROMETHEUS_CONTENT_TYPE_004)
                 .build();
         } finally {
             buf.release();