Produce a report for all the data collectors

Implement the API function of the monitoring daemon that provides the report
of all the data collectors.

Signed-off-by: Michele Tartara <mtartara@google.com>
Reviewed-by: Helga Velroyen <helgav@google.com>

src/Ganeti/Monitoring/Server.hs

@@ -139,7 +139,9 @@ reportHandler =
 
 -- | Return the report of all the available collectors.
 allReports :: Snap ()
-allReports = writeText "TODO: return the reports of all the collectors"
+allReports = do
+  reports <- mapM (liftIO . dReport) collectors
+  writeBS . pack . J.encode $ reports
 
 -- | Returns a category given its name.
 -- If "collector" is given as the name, the collector has no category, and