Implement ntopng plugins

Plugins are a convenient way to group together related lua scripts.
Their primary use case is to group user scripts and their alert/status
definition.
The builtin ntopng user scripts and definitions are now
packed into plugins directories. In future, we will support loading of
user created plugins.
Plugins are loaded at startup into some runtime directories and then
used. Other changes provided by this commit include:

- Add sample flow logger plugin
- Initial support for system user scripts
- Rename edge to threshold
- Migrate system probes to user scripts/plugins
- Migrate scripts to more explicit alerts_api.checkThresholdAlert api

scripts/plugins/influxdb_monitor/user_scripts/system/influxdb_monitor.lua

@@ -0,0 +1,100 @@
+--
+-- (C) 2019 - ntop.org
+--
+
+local ts_utils = require("ts_utils_core")
+
+local script = {
+  -- This module is enabled by default
+  default_enabled = true,
+
+  -- No default configuration is provided
+  default_value = {},
+
+  -- See below
+  hooks = {},
+}
+
+-- ##############################################
+
+function script.setup()
+  -- Only enabled if InfluxDB is active
+  return(ts_utils.getDriverName() == "influxdb")
+end
+
+-- ##############################################
+
+-- Defines an hook which is executed every 5 minutes
+script.hooks["5mins"] = function(params)
+  if params.ts_enabled then
+    local influxdb = ts_utils.getQueryDriver()
+
+    script._exportStats(when, influxdb)
+    script._measureRtt(when, influxdb)
+    script._exportStorageSize(when, influxdb)
+  end
+end
+
+-- ##############################################
+
+function script.getExportStats()
+   local points_exported
+   local points_dropped
+   local exports
+   local ifnames = interface.getIfNames()
+
+   local influxdb = ts_utils.getQueryDriver()
+
+   points_exported = influxdb:get_exported_points()
+   points_dropped = influxdb:get_dropped_points()
+   exports = influxdb:get_exports()
+
+   local res = {
+      health = influxdb:get_health(),
+      points_exported = points_exported,
+      points_dropped = points_dropped,
+      exports = exports,
+   }
+
+   return(res)
+end
+
+-- ##############################################
+
+function script._measureRtt(when, influxdb)
+   local start_ms = ntop.gettimemsec()
+   local res = influxdb:getInfluxdbVersion()
+   local ifid = getSystemInterfaceId()
+
+   if res ~= nil then
+      local end_ms = ntop.gettimemsec()
+
+      ts_utils.append("influxdb:rtt", {ifid = ifid, millis_rtt = ((end_ms-start_ms)*1000)}, when)
+   end
+end
+
+-- ##############################################
+
+function script._exportStats(when, influxdb)
+   local stats = script.getExportStats()
+   local ifid = getSystemInterfaceId()
+
+   ts_utils.append("influxdb:exported_points", {ifid = ifid, points = stats.points_exported}, when)
+   ts_utils.append("influxdb:dropped_points", {ifid = ifid, points = stats.points_dropped}, when)
+   ts_utils.append("influxdb:exports", {ifid = ifid, num_exports = stats.exports}, when)
+end
+
+-- ##############################################
+
+function script._exportStorageSize(when, influxdb)
+   local disk_bytes = influxdb:getDiskUsage()
+   local ifid = getSystemInterfaceId()
+
+   if(disk_bytes ~= nil) then
+      ts_utils.append("influxdb:storage_size", {ifid = ifid, disk_bytes = disk_bytes}, when)
+   end
+end
+
+-- ##############################################
+
+return(script)