MXS-1220: Add JSON return value to diagnostics entry points

The modules that implement a diagnostics entry point now return a JSON
type object. This removes the need to format data inside the modules.

The module implementations of these are not yet complete which means that
MaxScale will fail to compile.

include/maxscale/monitor.h

@@ -23,6 +23,7 @@
 #include <maxscale/config.h>
 #include <maxscale/dcb.h>
 #include <maxscale/server.h>
+#include <maxscale/jansson.h>
 
 MXS_BEGIN_DECLS
 
@@ -54,9 +55,38 @@ typedef struct mxs_monitor MXS_MONITOR;
  */
 typedef struct mxs_monitor_object
 {
+    /**
+     * @brief Start the monitor
+     *
+     * This entry point is called when the monitor is started. If the monitor
+     * requires polling of the servers, it should create a separate monitoring
+     * thread.
+     *
+     * @param monitor The monitor object
+     * @param params  Parameters for this monitor
+     *
+     * @return Pointer to the monitor specific data, stored in @c monitor->handle
+     */
     void *(*startMonitor)(MXS_MONITOR *monitor, const MXS_CONFIG_PARAMETER *params);
+
+    /**
+     * @brief Stop the monitor
+     *
+     * This entry point is called when the monitor is stopped. If the monitor
+     * uses a polling thread, the thread should be stopped.
+     *
+     * @param monitor The monitor object
+     */
     void (*stopMonitor)(MXS_MONITOR *monitor);
-    void (*diagnostics)(DCB *, const MXS_MONITOR *);
+
+    /**
+     * @brief Return diagnostic information about the monitor
+     *
+     * @return A JSON object representing the state of the monitor
+     *
+     * @see jansson.h
+     */
+    json_t* (*diagnostics)(const MXS_MONITOR *monitor);
 } MXS_MONITOR_OBJECT;
 
 /**