Fix predefined metrics

[NelsonVides]

When load-testing TLS, I found that the predefined connection metrics are missing in amoc_arsenal. To fix this, add the metrics to app.config in amoc-arsenal-xmpp:

   {amoc_arsenal,
    [{predefined_metrics, [{times, connection},
                           {counters, connections},
                           {counters, connection_retries},
                           {counters, connection_failures}]}
    ]}

Also, fix the calls to application:get_application(?MODULE) in amoc-arsenal:

{ok, App} = application:get_application(?MODULE)

Verify in a load test, that the metrics are working.

[chrzaszcz]

Looks good 👌