Improve dropped alerts information

- Add informative label
- Add dropped alerts timeseries and chart
- Generate a system alert whenever alerts are dropped on a network interface
- Add a periodic activity warning when some alerts are dropped on that activity

Closes #3025

scripts/plugins/alerts_drops/user_scripts/system/dropped_alerts.lua

@@ -0,0 +1,58 @@
+--
+-- (C) 2019-20 - ntop.org
+--
+
+local user_scripts = require("user_scripts")
+local alerts_api = require("alerts_api")
+local alert_consts = require("alert_consts")
+
+local script = {
+  -- Script category
+  category = user_scripts.script_categories.internals,
+
+  -- This script is only for alerts generation
+  is_alert = true,
+
+  -- See below
+  hooks = {},
+
+  gui = {
+    i18n_title = "alerts_dashboard.alert_drops",
+    i18n_description = "alerts_dashboard.alert_drops_description",
+  },
+}
+
+-- #################################################################
+
+function script.hooks.min(params)
+  local available_interfaces = interface.getIfNames()
+
+  for _, iface in pairs(available_interfaces) do
+    interface.select(iface)
+
+    local new_dropped_alerts = interface.checkDroppedAlerts()
+
+    local alert_type = {
+      alert_type = alert_consts.alert_types.alert_dropped_alerts,
+      alert_granularity = alert_consts.alerts_granularities.min,
+      alert_severity = alert_consts.alert_severities.error,
+      alert_type_params = {
+        ifid = interface.getId(),
+        num_dropped = new_dropped_alerts,
+      }
+    }
+
+    -- Note: required for the trigger/release below
+    interface.select(getSystemInterfaceId())
+
+    if(new_dropped_alerts > 0) then
+      alerts_api.trigger(params.alert_entity, alert_type, nil, params.cur_alerts)
+    else
+      alerts_api.release(params.alert_entity, alert_type, nil, params.cur_alerts)
+    end
+  end
+end
+
+-- #################################################################
+
+return script