Add OpenTelemetry Prometheus metrics with configurable port/host

pom.xml

@@ -383,12 +383,44 @@
     <jetty.version>11.0.26</jetty.version>
     <modernizer.version>3.2.0</modernizer.version>
     <njord.version>0.7.5</njord.version>
+    <opentelemetry.version>1.57.0</opentelemetry.version>
+    <opentelemetry-semconv.version>1.25.0-alpha</opentelemetry-semconv.version>
     <slf4j.version>2.0.17</slf4j.version>
     <shade.prefix>${project.groupId}.shaded</shade.prefix>
     <surefire.version>3.5.2</surefire.version>
   </properties>
 
+  <dependencyManagement>
+    <dependencies>
+      <dependency>
+        <groupId>io.opentelemetry</groupId>
+        <artifactId>opentelemetry-bom</artifactId>
+        <version>${opentelemetry.version}</version>
+        <type>pom</type>
+        <scope>import</scope>
+      </dependency>
+    </dependencies>
+  </dependencyManagement>
+
   <dependencies>
+    <dependency>
+      <groupId>io.opentelemetry</groupId>
+      <artifactId>opentelemetry-api</artifactId>
+    </dependency>
+    <dependency>
+      <groupId>io.opentelemetry</groupId>
+      <artifactId>opentelemetry-sdk</artifactId>
+    </dependency>
+    <dependency>
+      <groupId>io.opentelemetry</groupId>
+      <artifactId>opentelemetry-exporter-prometheus</artifactId>
+      <version>1.45.0-alpha</version>
+    </dependency>
+    <dependency>
+      <groupId>io.opentelemetry.semconv</groupId>
+      <artifactId>opentelemetry-semconv</artifactId>
+      <version>${opentelemetry-semconv.version}</version>
+    </dependency>
     <dependency>
       <groupId>com.amazonaws</groupId>
       <artifactId>aws-java-sdk-s3</artifactId>

src/main/java/org/gaul/s3proxy/MetricsHandler.java

@@ -0,0 +1,48 @@
+/*
+ * Copyright 2014-2025 Andrew Gaul <andrew@gaul.org>
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.gaul.s3proxy;
+
+import java.io.IOException;
+
+import jakarta.servlet.http.HttpServletRequest;
+import jakarta.servlet.http.HttpServletResponse;
+
+import org.eclipse.jetty.server.Request;
+import org.eclipse.jetty.server.handler.AbstractHandler;
+
+/** Jetty handler that serves Prometheus metrics at /metrics endpoint. */
+public final class MetricsHandler extends AbstractHandler {
+    private final S3ProxyMetrics metrics;
+
+    public MetricsHandler(S3ProxyMetrics metrics) {
+        this.metrics = metrics;
+    }
+
+    @Override
+    public void handle(String target, Request baseRequest,
+            HttpServletRequest request, HttpServletResponse response)
+            throws IOException {
+        if (!"/metrics".equals(target)) {
+            return;
+        }
+
+        response.setContentType("text/plain; version=0.0.4; charset=utf-8");
+        response.setStatus(HttpServletResponse.SC_OK);
+        response.getWriter().write(metrics.scrape());
+        baseRequest.setHandled(true);
+    }
+}

src/main/java/org/gaul/s3proxy/S3Operation.java

@@ -0,0 +1,57 @@
+/*
+ * Copyright 2014-2025 Andrew Gaul <andrew@gaul.org>
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.gaul.s3proxy;
+
+/** Enumeration of S3 operations for metrics tracking. */
+public enum S3Operation {
+    LIST_BUCKETS("ListBuckets"),
+    LIST_OBJECTS_V2("ListObjectsV2"),
+    GET_OBJECT("GetObject"),
+    PUT_OBJECT("PutObject"),
+    DELETE_OBJECT("DeleteObject"),
+    DELETE_OBJECTS("DeleteObjects"),
+    CREATE_BUCKET("CreateBucket"),
+    DELETE_BUCKET("DeleteBucket"),
+    HEAD_BUCKET("HeadBucket"),
+    HEAD_OBJECT("HeadObject"),
+    COPY_OBJECT("CopyObject"),
+    CREATE_MULTIPART_UPLOAD("CreateMultipartUpload"),
+    UPLOAD_PART("UploadPart"),
+    UPLOAD_PART_COPY("UploadPartCopy"),
+    COMPLETE_MULTIPART_UPLOAD("CompleteMultipartUpload"),
+    ABORT_MULTIPART_UPLOAD("AbortMultipartUpload"),
+    LIST_MULTIPART_UPLOADS("ListMultipartUploads"),
+    LIST_PARTS("ListParts"),
+    GET_OBJECT_ACL("GetObjectAcl"),
+    PUT_OBJECT_ACL("PutObjectAcl"),
+    GET_BUCKET_ACL("GetBucketAcl"),
+    PUT_BUCKET_ACL("PutBucketAcl"),
+    GET_BUCKET_LOCATION("GetBucketLocation"),
+    GET_BUCKET_POLICY("GetBucketPolicy"),
+    OPTIONS_OBJECT("OptionsObject"),
+    UNKNOWN("Unknown");
+
+    private final String value;
+
+    S3Operation(String value) {
+        this.value = value;
+    }
+
+    public String getValue() {
+        return value;
+    }
+}

src/main/java/org/gaul/s3proxy/S3Proxy.java

@@ -40,6 +40,7 @@
 import org.eclipse.jetty.server.Server;
 import org.eclipse.jetty.server.ServerConnector;
 import org.eclipse.jetty.server.handler.ContextHandler;
+import org.eclipse.jetty.server.handler.HandlerList;
 import org.eclipse.jetty.util.ssl.SslContextFactory;
 import org.eclipse.jetty.util.thread.QueuedThreadPool;
 import org.jclouds.blobstore.BlobStore;
@@ -53,6 +54,7 @@
 public final class S3Proxy {
     private final Server server;
     private final S3ProxyHandlerJetty handler;
+    private final S3ProxyMetrics metrics;
     private final boolean listenHTTP;
     private final boolean listenHTTPS;
 
@@ -128,14 +130,28 @@ public final class S3Proxy {
         } else {
             listenHTTPS = false;
         }
+        if (builder.metricsEnabled) {
+            this.metrics = new S3ProxyMetrics(
+                    builder.metricsHost, builder.metricsPort);
+        } else {
+            this.metrics = null;
+        }
+
         handler = new S3ProxyHandlerJetty(builder.blobStore,
                 builder.authenticationType, builder.identity,
                 builder.credential, builder.virtualHost,
                 builder.maxSinglePartObjectSize,
                 builder.v4MaxNonChunkedRequestSize,
                 builder.ignoreUnknownHeaders, builder.corsRules,
-                builder.servicePath, builder.maximumTimeSkew);
-        server.setHandler(handler);
+                builder.servicePath, builder.maximumTimeSkew, metrics);
+
+        if (metrics != null) {
+            var metricsHandler = new MetricsHandler(metrics);
+            var handlerList = new HandlerList(metricsHandler, handler);
+            server.setHandler(handlerList);
+        } else {
+            server.setHandler(handler);
+        }
     }
 
     public static final class Builder {
@@ -157,6 +173,9 @@ public static final class Builder {
         private CrossOriginResourceSharing corsRules;
         private int jettyMaxThreads = 200;  // sourced from QueuedThreadPool()
         private int maximumTimeSkew = 15 * 60;
+        private boolean metricsEnabled;
+        private int metricsPort = S3ProxyMetrics.DEFAULT_METRICS_PORT;
+        private String metricsHost = S3ProxyMetrics.DEFAULT_METRICS_HOST;
 
         Builder() {
         }
@@ -321,6 +340,24 @@ public static Builder fromProperties(Properties properties)
                 builder.maximumTimeSkew(Integer.parseInt(maximumTimeSkew));
             }
 
+            String metricsEnabled = properties.getProperty(
+                    S3ProxyConstants.PROPERTY_METRICS_ENABLED);
+            if (!Strings.isNullOrEmpty(metricsEnabled)) {
+                builder.metricsEnabled(Boolean.parseBoolean(metricsEnabled));
+            }
+
+            String metricsPort = properties.getProperty(
+                    S3ProxyConstants.PROPERTY_METRICS_PORT);
+            if (!Strings.isNullOrEmpty(metricsPort)) {
+                builder.metricsPort(Integer.parseInt(metricsPort));
+            }
+
+            String metricsHost = properties.getProperty(
+                    S3ProxyConstants.PROPERTY_METRICS_HOST);
+            if (!Strings.isNullOrEmpty(metricsHost)) {
+                builder.metricsHost(metricsHost);
+            }
+
             return builder;
         }
 
@@ -409,6 +446,21 @@ public Builder maximumTimeSkew(int maximumTimeSkew) {
             return this;
         }
 
+        public Builder metricsEnabled(boolean metricsEnabled) {
+            this.metricsEnabled = metricsEnabled;
+            return this;
+        }
+
+        public Builder metricsPort(int metricsPort) {
+            this.metricsPort = metricsPort;
+            return this;
+        }
+
+        public Builder metricsHost(String metricsHost) {
+            this.metricsHost = requireNonNull(metricsHost);
+            return this;
+        }
+
         public Builder servicePath(String s3ProxyServicePath) {
             String path = Strings.nullToEmpty(s3ProxyServicePath);
 
@@ -487,6 +539,9 @@ public void start() throws Exception {
 
     public void stop() throws Exception {
         server.stop();
+        if (metrics != null) {
+            metrics.close();
+        }
     }
 
     public int getPort() {

src/main/java/org/gaul/s3proxy/S3ProxyConstants.java

@@ -144,6 +144,15 @@ public final class S3ProxyConstants {
     public static final String PROPERTY_NO_CACHE_BLOBSTORE =
             "s3proxy.no-cache-blobstore";
 
+    /** Enable Prometheus metrics endpoint at /metrics. */
+    public static final String PROPERTY_METRICS_ENABLED =
+            "s3proxy.metrics.enabled";
+
+    public static final String PROPERTY_METRICS_PORT =
+            "s3proxy.metrics.port";
+    public static final String PROPERTY_METRICS_HOST =
+            "s3proxy.metrics.host";
+
     static final String PROPERTY_ALT_JCLOUDS_PREFIX = "alt.";
 
     private S3ProxyConstants() {