SOLR-10654: Introduce output of Prometheus metrics directly from Solr

solr/core/build.gradle

@@ -153,6 +153,13 @@ dependencies {
   implementation 'org.apache.logging.log4j:log4j-core'
   runtimeOnly 'org.apache.logging.log4j:log4j-slf4j2-impl'
 
+  // For the PrometheusResponseWriter
+  implementation 'io.prometheus:prometheus-metrics-model:1.1.0'
+  implementation('io.prometheus:prometheus-metrics-exposition-formats:1.1.0', {
+    exclude group: "io.prometheus", module: "prometheus-metrics-shaded-protobuf"
+    exclude group: "io.prometheus", module: "prometheus-metrics-config"
+  })
+
   // For faster XML processing than the JDK
   implementation 'org.codehaus.woodstox:stax2-api'
   implementation 'com.fasterxml.woodstox:woodstox-core'

solr/core/src/java/org/apache/solr/core/SolrCore.java

@@ -17,6 +17,7 @@
 package org.apache.solr.core;
 
 import static org.apache.solr.common.params.CommonParams.PATH;
+import static org.apache.solr.handler.admin.MetricsHandler.PROMETHEUS_METRICS_WT;
 
 import com.codahale.metrics.Counter;
 import com.codahale.metrics.Timer;
@@ -130,6 +131,7 @@
 import org.apache.solr.response.JacksonJsonWriter;
 import org.apache.solr.response.PHPResponseWriter;
 import org.apache.solr.response.PHPSerializedResponseWriter;
+import org.apache.solr.response.PrometheusResponseWriter;
 import org.apache.solr.response.PythonResponseWriter;
 import org.apache.solr.response.QueryResponseWriter;
 import org.apache.solr.response.RawResponseWriter;
@@ -3032,6 +3034,7 @@ public PluginBag<QueryResponseWriter> getResponseWriters() {
     m.put("csv", new CSVResponseWriter());
     m.put("schema.xml", new SchemaXmlResponseWriter());
     m.put("smile", new SmileResponseWriter());
+    m.put(PROMETHEUS_METRICS_WT, new PrometheusResponseWriter());
     m.put(ReplicationHandler.FILE_STREAM, getFileStreamWriter());
     DEFAULT_RESPONSE_WRITERS = Collections.unmodifiableMap(m);
     try {

solr/core/src/java/org/apache/solr/handler/admin/MetricsHandler.java

@@ -40,6 +40,7 @@
 import java.util.stream.Collectors;
 import org.apache.solr.common.MapWriter;
 import org.apache.solr.common.SolrException;
+import org.apache.solr.common.params.CommonParams;
 import org.apache.solr.common.params.SolrParams;
 import org.apache.solr.common.util.CommonTestInjection;
 import org.apache.solr.common.util.NamedList;
@@ -50,6 +51,7 @@
 import org.apache.solr.metrics.SolrMetricManager;
 import org.apache.solr.request.SolrQueryRequest;
 import org.apache.solr.request.SolrRequestInfo;
+import org.apache.solr.response.PrometheusResponseWriter;
 import org.apache.solr.response.SolrQueryResponse;
 import org.apache.solr.security.AuthorizationContext;
 import org.apache.solr.security.PermissionNameProvider;
@@ -68,6 +70,7 @@ public class MetricsHandler extends RequestHandlerBase implements PermissionName
   public static final String KEY_PARAM = "key";
   public static final String EXPR_PARAM = "expr";
   public static final String TYPE_PARAM = "type";
+  public static final String PROMETHEUS_METRICS_WT = "prometheus";
 
   public static final String ALL = "all";
 
@@ -116,13 +119,19 @@ public void handleRequestBody(SolrQueryRequest req, SolrQueryResponse rsp) throw
     }
   }
 
-  private void handleRequest(SolrParams params, BiConsumer<String, Object> consumer)
-      throws Exception {
+  private void handleRequest(SolrParams params, BiConsumer<String, Object> consumer) {
+    NamedList<Object> response;
+
     if (!enabled) {
       consumer.accept("error", "metrics collection is disabled");
       return;
     }
-    boolean compact = params.getBool(COMPACT_PARAM, true);
+
+    if (PROMETHEUS_METRICS_WT.equals(params.get(CommonParams.WT))) {
+      response = handlePrometheusExport(params);
+      consumer.accept("metrics", response);
+      return;
+    }
     String[] keys = params.getParams(KEY_PARAM);
     if (keys != null && keys.length > 0) {
       handleKeyRequest(keys, consumer);
@@ -133,6 +142,14 @@ private void handleRequest(SolrParams params, BiConsumer<String, Object> consume
       handleExprRequest(exprs, consumer);
       return;
     }
+
+    response = handleDropwizardRegistry(params);
+
+    consumer.accept("metrics", response);
+  }
+
+  private NamedList<Object> handleDropwizardRegistry(SolrParams params) {
+    boolean compact = params.getBool(COMPACT_PARAM, true);
     MetricFilter mustMatchFilter = parseMustMatchFilter(params);
     Predicate<CharSequence> propertyFilter = parsePropertyFilter(params);
     List<MetricType> metricTypes = parseMetricTypes(params);
@@ -144,6 +161,7 @@ private void handleRequest(SolrParams params, BiConsumer<String, Object> consume
     for (String registryName : requestedRegistries) {
       MetricRegistry registry = metricManager.registry(registryName);
       SimpleOrderedMap<Object> result = new SimpleOrderedMap<>();
+
       MetricUtils.toMaps(
           registry,
           metricFilters,
@@ -158,7 +176,34 @@ private void handleRequest(SolrParams params, BiConsumer<String, Object> consume
         response.add(registryName, result);
       }
     }
-    consumer.accept("metrics", response);
+    return response;
+  }
+
+  private NamedList<Object> handlePrometheusExport(SolrParams params) {
+    NamedList<Object> response = new SimpleOrderedMap<>();
+    MetricFilter mustMatchFilter = parseMustMatchFilter(params);
+    Predicate<CharSequence> propertyFilter = parsePropertyFilter(params);
+    List<MetricType> metricTypes = parseMetricTypes(params);
+    List<MetricFilter> metricFilters =
+        metricTypes.stream().map(MetricType::asMetricFilter).collect(Collectors.toList());
+    Set<String> requestedRegistries = parseRegistries(params);
+
+    for (String registryName : requestedRegistries) {
+      MetricRegistry dropwizardRegistry = metricManager.registry(registryName);
+      PrometheusResponseWriter.toPrometheus(
+          dropwizardRegistry,
+          registryName,
+          metricFilters,
+          mustMatchFilter,
+          propertyFilter,
+          false,
+          false,
+          true,
+          (registry) -> {
+            response.add(registryName, registry);
+          });
+    }
+    return response;
   }
 
   private static class MetricsExpr {

solr/core/src/java/org/apache/solr/metrics/prometheus/SolrMetric.java

@@ -0,0 +1,57 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You 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
+ *
+ *     http://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.apache.solr.metrics.prometheus;
+
+import com.codahale.metrics.Metric;
+import io.prometheus.metrics.model.snapshots.Labels;
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.Map;
+
+/**
+ * Base class is a wrapper to categorize and export {@link com.codahale.metrics.Metric} to {@link
+ * io.prometheus.metrics.model.snapshots.DataPointSnapshot} and register to a {@link
+ * SolrPrometheusExporter}. {@link com.codahale.metrics.MetricRegistry} does not support tags unlike
+ * prometheus. Metrics registered to the registry need to be parsed out from the metric name to be
+ * exported to {@link io.prometheus.metrics.model.snapshots.DataPointSnapshot}
+ */
+public abstract class SolrMetric {
+  public Metric dropwizardMetric;
+  public String metricName;
+  public Map<String, String> labels = new HashMap<>();
+
+  public SolrMetric() {}
+
+  public SolrMetric(Metric dropwizardMetric, String metricName) {
+    this.dropwizardMetric = dropwizardMetric;
+    this.metricName = metricName;
+  }
+
+  /*
+   * Parse labels from the Dropwizard Metric name to be exported
+   */
+  public abstract SolrMetric parseLabels();
+
+  /*
+   * Export metric to Prometheus with labels
+   */
+  public abstract void toPrometheus(SolrPrometheusExporter exporter);
+
+  public Labels getLabels() {
+    return Labels.of(new ArrayList<>(labels.keySet()), new ArrayList<>(labels.values()));
+  }
+}

solr/core/src/java/org/apache/solr/metrics/prometheus/SolrNoOpMetric.java

@@ -0,0 +1,29 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You 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
+ *
+ *     http://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.apache.solr.metrics.prometheus;
+
+public class SolrNoOpMetric extends SolrMetric {
+  public SolrNoOpMetric() {}
+
+  @Override
+  public SolrMetric parseLabels() {
+    return this;
+  }
+
+  @Override
+  public void toPrometheus(SolrPrometheusExporter exporter) {}
+}