Add explicit system module to avoid getting start/stop called twice

Due to the bug 2106 a LifecycleListener's start/stop method are called
twice. As documented in 2106, this only happens when there are no
explicitly defined modules and modules are automatically generated.

Change-Id: I1ed0f7f855401de9410785664566c5d9e8edae3e

BUILD

diff --git a/BUILD b/BUILD
index 1013c5e..9fbbd36 100644
--- a/BUILD
+++ b/BUILD
@@ -5,6 +5,7 @@
     srcs = glob(["src/main/java/**/*.java"]),
     manifest_entries = [
         "Gerrit-PluginName: metrics-reporter-jmx",
+        "Gerrit-Module: com.googlesource.gerrit.plugins.metricsreporterjmx.Module",
     ],
     resources = glob(["src/main/resources/**/*"]),
     deps = ["@metrics_jmx//jar"],

src/main/java/com/googlesource/gerrit/plugins/metricsreporterjmx/Module.java

diff --git a/src/main/java/com/googlesource/gerrit/plugins/metricsreporterjmx/Module.java b/src/main/java/com/googlesource/gerrit/plugins/metricsreporterjmx/Module.java
new file mode 100644
index 0000000..aa9d1df
--- /dev/null
+++ b/src/main/java/com/googlesource/gerrit/plugins/metricsreporterjmx/Module.java
@@ -0,0 +1,24 @@
+// Copyright (C) 2020 The Android Open Source Project
+//
+// 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
+//
+// 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 com.googlesource.gerrit.plugins.metricsreporterjmx;
+
+import com.google.gerrit.lifecycle.LifecycleModule;
+
+public class Module extends LifecycleModule {
+  @Override
+  protected void configure() {
+    listener().to(GerritJmxReporter.class);
+  }
+}