expose Prometheus metrics through QRest

modules/qrest/src/dist/deploy/30_qrest_txnmgr.xml

@@ -7,6 +7,7 @@
   <participant class="org.jpos.qrest.participant.Router">
     <route path="/q2**" method="GET" name="q2"/>
     <route path="/q2**" method="POST" name="q2"/>
+    <route path="/metrics" method="GET" name="metrics"/>
     <route path="/test/load_file" method="POST" name="upload_file"/>
     <route path="/welcome.html" method="GET" name="welcome" />
     <route path="/dynamic" method="GET" name="dynamic" />
@@ -17,6 +18,9 @@
   <group name="q2">
     <participant class="org.jpos.qrest.participant.Q2Info" />
   </group>
+  <group name="metrics">
+    <participant class="org.jpos.qrest.participant.Metrics" />
+  </group>
   <group name="welcome">
     <participant class="org.jpos.qrest.participant.StaticContent">
         <property name="documentRoot" value="html" />

modules/qrest/src/main/java/org/jpos/qrest/participant/Metrics.java

@@ -0,0 +1,61 @@
+/*
+ * jPOS Project [http://jpos.org]
+ * Copyright (C) 2000-2021 jPOS Software SRL
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as
+ * published by the Free Software Foundation, either version 3 of the
+ * License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+package org.jpos.qrest.participant;
+
+import java.io.Serializable;
+import java.util.*;
+
+import io.netty.buffer.Unpooled;
+import io.netty.handler.codec.http.*;
+import org.jpos.q2.Q2;
+import org.jpos.qrest.Route;
+import org.jpos.transaction.Context;
+import org.jpos.transaction.TransactionManager;
+import org.jpos.transaction.TransactionParticipant;
+
+import static io.netty.handler.codec.http.HttpResponseStatus.OK;
+import static io.netty.handler.codec.http.HttpVersion.HTTP_1_1;
+import static org.jpos.qrest.Constants.RESPONSE;
+
+public class Metrics implements TransactionParticipant {
+    // special temp key in response map to indicate a specific HttpResponseStatus value (should be removed from actual response)
+    private final static String HTTP_STATUS_KEY = "__http_status__";
+    private TransactionManager txnmgr;
+    private Q2 q2;
+    private List<Route<Map<String,Object>>> routes = new ArrayList<>();
+    private String prefix;
+
+    public Metrics () { }
+
+    @Override
+    public int prepare(long id, Serializable context) {
+        Context ctx = (Context) context;
+        HttpResponse response = new DefaultFullHttpResponse(HTTP_1_1, OK,
+          Unpooled.wrappedBuffer(q2.getPrometheusMeterRegistry().scrape().getBytes())
+        );
+        ctx.put(RESPONSE, response);
+        return PREPARED | NO_JOIN | READONLY;
+    }
+
+    @SuppressWarnings("unused")
+    public void setTransactionManager (TransactionManager txnmgr) {
+        this.txnmgr = txnmgr;
+        this.q2 = txnmgr.getServer();
+    }
+}