SOLR-16738: Refactor AdminHandlersProxy for better extensibility

solr/api/src/java/org/apache/solr/client/api/model/NodeSystemResponse.java

@@ -16,6 +16,8 @@
  */
 package org.apache.solr.client.api.model;
 
+import com.fasterxml.jackson.annotation.JsonAnyGetter;
+import com.fasterxml.jackson.annotation.JsonAnySetter;
 import com.fasterxml.jackson.annotation.JsonProperty;
 import java.util.Date;
 import java.util.List;
@@ -25,6 +27,20 @@
 /** Response from /node/system */
 public class NodeSystemResponse extends SolrJerseyResponse {
 
+  // TODO The typing here is kindof wonky - can I tighten 'Object' here to be NodeSystemResponse or
+  // will Jackson choke on that?
+  public Map<String, Object> remoteNodeData;
+
+  @JsonAnyGetter
+  public Map<String, Object> remoteNodeData() {
+    return remoteNodeData;
+  }
+
+  @JsonAnySetter
+  public void setRemoteNodeResponse(String field, Object value) {
+    remoteNodeData.put(field, value);
+  }
+
   @JsonProperty public String host;
   @JsonProperty public String node;
   @JsonProperty public String mode;

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

@@ -31,6 +31,7 @@
 import org.apache.solr.core.CoreContainer;
 import org.apache.solr.handler.RequestHandlerBase;
 import org.apache.solr.handler.admin.api.NodeLogging;
+import org.apache.solr.handler.admin.proxy.GenericV1RequestProxy;
 import org.apache.solr.handler.api.V2ApiUtils;
 import org.apache.solr.logging.LogWatcher;
 import org.apache.solr.request.SolrQueryRequest;
@@ -91,9 +92,7 @@ public void handleRequestBody(SolrQueryRequest req, SolrQueryResponse rsp) throw
     }
 
     rsp.setHttpCaching(false);
-    if (cc != null && AdminHandlersProxy.maybeProxyToNodes(req, rsp, cc)) {
-      return; // Request was proxied to other node
-    }
+    new GenericV1RequestProxy(cc, req, rsp).proxyRequest();
   }
 
   private void squashV2Response(SolrQueryResponse rsp, LoggingResponse response) {

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

@@ -17,6 +17,8 @@
 
 package org.apache.solr.handler.admin;
 
+import static org.apache.solr.common.params.CommonParams.METRICS_PATH;
+
 import io.prometheus.metrics.model.snapshots.MetricSnapshot;
 import io.prometheus.metrics.model.snapshots.MetricSnapshots;
 import java.util.ArrayList;
@@ -26,12 +28,17 @@
 import java.util.SortedMap;
 import java.util.function.BiConsumer;
 import org.apache.solr.api.JerseyResource;
+import org.apache.solr.client.solrj.SolrRequest;
+import org.apache.solr.client.solrj.response.InputStreamResponseParser;
 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.NamedList;
 import org.apache.solr.core.CoreContainer;
 import org.apache.solr.handler.RequestHandlerBase;
 import org.apache.solr.handler.admin.api.GetMetrics;
+import org.apache.solr.handler.admin.proxy.GenericV1RequestProxy;
+import org.apache.solr.handler.admin.proxy.RemoteRequestProxy;
 import org.apache.solr.metrics.SolrMetricManager;
 import org.apache.solr.metrics.otel.FilterablePrometheusMetricReader;
 import org.apache.solr.request.SolrQueryRequest;
@@ -101,9 +108,12 @@ public void handleRequestBody(SolrQueryRequest req, SolrQueryResponse rsp) throw
               + format);
     }
 
-    if (cc != null && AdminHandlersProxy.maybeProxyToNodes(req, rsp, cc)) {
+    final var reqProxy = createMetricProxy(cc, req, rsp);
+    if (cc != null && reqProxy.shouldProxy()) {
+      reqProxy.proxyRequest();
       return; // Request was proxied to other node
     }
+
     SolrRequestInfo.setRequestInfo(new SolrRequestInfo(req, rsp));
     try {
       handleRequest(req.getParams(), (k, v) -> rsp.add(k, v));
@@ -143,6 +153,37 @@ public void handleRequest(SolrParams params, BiConsumer<String, Object> consumer
     consumer.accept("metrics", mergedSnapshots);
   }
 
+  public static RemoteRequestProxy createMetricProxy(
+      CoreContainer cc, SolrQueryRequest req, SolrQueryResponse rsp) {
+    return new GenericV1RequestProxy(cc, req, rsp) {
+
+      // Metric requests use 'node' to proxy rather than the generally accepted "nodes"
+      @Override
+      protected String getDestinationNodeParamName() {
+        return "node";
+      }
+
+      // Metrics requests require a particular ResponseParser
+      @Override
+      protected SolrRequest<?> createGenericRequest(String apiPath, SolrParams params) {
+        final var toProxy = super.createGenericRequest(apiPath, params);
+        // Metrics proxy might be called from either v1 or v2, but end up proxying to v1 for
+        // simplicity
+        toProxy.setPath(METRICS_PATH);
+        String wt = params.get(CommonParams.WT, MetricUtils.PROMETHEUS_METRICS_WT);
+        toProxy.setResponseParser(new InputStreamResponseParser(wt));
+
+        return toProxy;
+      }
+
+      // Metrics requests only proxy to single host, so proxied response is added at root level
+      @Override
+      public void processProxiedResponse(String nodeName, NamedList<Object> proxiedResponse) {
+        rsp.getValues().addAll(proxiedResponse);
+      }
+    };
+  }
+
   @Override
   public String getDescription() {
     return "A handler to return all the metrics gathered by Solr";