MXS-1848 Introduce maxscape::MonitorApi

The purpose of this template class is to provide the
implementation of the monitor C-api. It's to be instantiated
with the class that actually provides the monitor behaviour.

A separate class maxscale::MonitorInstance will be provided
that then in turn implements the behaviour common to most
monitors. So, the structure will be like:

    class SomeMonitor : public maxscale::MonitorInstance {...}

    extern "C" MXS_MODULE* MXS_CREATE_MODULE()
    {
        static MXS_MODULE info =
        {
            ...
            &maxscale::MonitorApi<SomeMonitor>::s_api,
            ...
        };

        return &info;
    }

include/maxscale/monitor.hh

@@ -0,0 +1,85 @@
+#pragma once
+/*
+ * Copyright (c) 2016 MariaDB Corporation Ab
+ *
+ * Use of this software is governed by the Business Source License included
+ * in the LICENSE.TXT file and at www.mariadb.com/bsl11.
+ *
+ * Change Date: 2020-01-01
+ *
+ * On the date above, in accordance with the Business Source License, use
+ * of this software will be governed by version 2 or later of the General
+ * Public License.
+ */
+
+#include <maxscale/cppdefs.hh>
+#include <maxscale/monitor.h>
+
+namespace maxscale
+{
+
+/**
+ * The purpose of the template MonitorApi is to provide an implementation
+ * of the monitor C-API. The template is instantiated with a class that
+ * provides the actual behaviour of a monitor.
+ */
+template<class MonitorInstance>
+class MonitorApi
+{
+public:
+    MonitorApi() = delete;
+    MonitorApi(const MonitorApi&) = delete;
+    MonitorApi& operator = (const MonitorApi&) = delete;
+
+    static MXS_MONITOR_INSTANCE* createInstance(MXS_MONITOR* pMonitor)
+    {
+        MonitorInstance* pInstance = NULL;
+        MXS_EXCEPTION_GUARD(pInstance = MonitorInstance::create(pMonitor));
+        return pInstance;
+    }
+
+    static void destroyInstance(MXS_MONITOR_INSTANCE* pInstance)
+    {
+        MXS_EXCEPTION_GUARD(MonitorInstance::destroy(static_cast<MonitorInstance*>(pInstance)));
+    }
+
+    static bool startMonitor(MXS_MONITOR_INSTANCE* pInstance,
+                             const MXS_CONFIG_PARAMETER* pParams)
+    {
+        bool started = false;
+        MXS_EXCEPTION_GUARD(started = static_cast<MonitorInstance*>(pInstance)->start(pParams));
+        return started;
+    }
+
+    static void stopMonitor(MXS_MONITOR_INSTANCE* pInstance)
+    {
+        MXS_EXCEPTION_GUARD(static_cast<MonitorInstance*>(pInstance)->stop());
+    }
+
+    static void diagnostics(const MXS_MONITOR_INSTANCE* pInstance, DCB* pDcb)
+    {
+        MXS_EXCEPTION_GUARD(static_cast<const MonitorInstance*>(pInstance)->diagnostics(pDcb));
+    }
+
+    static json_t* diagnostics_json(const MXS_MONITOR_INSTANCE* pInstance)
+    {
+        json_t* pJson = NULL;
+        MXS_EXCEPTION_GUARD(pJson = static_cast<const MonitorInstance*>(pInstance)->diagnostics_json());
+        return pJson;
+    }
+
+    static MXS_MONITOR_API s_api;
+};
+
+template<class MonitorInstance>
+MXS_MONITOR_API MonitorApi<MonitorInstance>::s_api =
+{
+    &MonitorApi<MonitorInstance>::createInstance,
+    &MonitorApi<MonitorInstance>::destroyInstance,
+    &MonitorApi<MonitorInstance>::startMonitor,
+    &MonitorApi<MonitorInstance>::stopMonitor,
+    &MonitorApi<MonitorInstance>::diagnostics,
+    &MonitorApi<MonitorInstance>::diagnostics_json,
+};
+
+}