Rewrite statistics export

Add automatic export of metrics from slaves to the master server.  And
rewrite the statistics export plugin to use this new system.

plugins/statistics_exporter/info.js

@@ -0,0 +1,9 @@
+let link = require("lib/link");
+
+module.exports = {
+	name: "statistics_exporter",
+	title: "Prometheus Statistics Export",
+	description: "Exports statistics to Prometheus",
+	version: "2.0.0-alpha",
+	instanceEntrypoint: "instance",
+}

plugins/statistics_exporter/instance.js

@@ -0,0 +1,62 @@
+const plugin = require('lib/plugin');
+const { Gauge } = require('lib/prometheus');
+
+
+const instancePlayerCount = new Gauge(
+	'clusterio_instance_player_count', "Amount of players connected to this cluster",
+	{ labels: ['instance_id'] }
+);
+const instanceGameTicksTotal = new Gauge(
+	'clusterio_instance_game_ticks_total', "Game tick an instance has progressed to",
+	{ labels: ['instance_id'] }
+);
+const instanceForceFlowStatistics = new Gauge(
+	'clusterio_instance_force_flow_statistics', "Items/fluids/enemies/buildings produced/built/killed by a force",
+	{ labels: ['instance_id', 'force', 'statistic', 'direction', 'name'] },
+);
+const instanceGameFlowStatistics = new Gauge(
+	'clusterio_instance_game_flow_statistics', "Pollution produced/consumed in the game",
+	{ labels: ['instance_id', 'statistic', 'direction', 'name'] },
+);
+
+
+class InstancePlugin extends plugin.BaseInstancePlugin {
+	async onMetrics() {
+		let string = await this.instance.server.sendRcon('/sc remote.call("statistics_exporter", "export")');
+		let stats =  JSON.parse(string);
+
+		let instanceId = this.instance.config.id;
+		instanceGameTicksTotal.labels(String(instanceId)).set(stats.game_tick);
+		instancePlayerCount.labels(String(instanceId)).set(stats.player_count);
+
+		for (let [forceName, flowStatistics] of Object.entries(stats.force_flow_statistics)) {
+			for (let [statisticName, statistic] of Object.entries(flowStatistics)) {
+				for (let [direction, counts] of Object.entries(statistic)) {
+					for (let [item, value] of Object.entries(counts)) {
+						instanceForceFlowStatistics.labels(
+							String(instanceId), forceName, statisticName, direction, item
+						).set(value);
+					}
+				}
+			}
+		}
+
+		for (let [direction, counts] of Object.entries(stats.game_flow_statistics.pollution_statistics)) {
+			for (let [item, value] of Object.entries(counts)) {
+				instanceGameFlowStatistics.labels(
+					String(instanceId), "pollution_statistics", direction, item
+				).set(value);
+			}
+		}
+	}
+}
+
+module.exports = {
+	InstancePlugin,
+
+	// For testing only
+	_instancePlayerCount: instancePlayerCount,
+	_instanceGameTicksTotal: instanceGameTicksTotal,
+	_instanceForceFlowStatistics: instanceForceFlowStatistics,
+	_instanceGameFlowStatistics: instanceGameFlowStatistics,
+}

plugins/statistics_exporter/lua/export.lua

@@ -0,0 +1,39 @@
+local statistics = {
+    "item_production_statistics",
+    "fluid_production_statistics",
+    "kill_count_statistics",
+    "entity_build_count_statistics",
+}
+
+local function export()
+    local stats = {
+        game_tick = game.tick,
+        player_count = #game.connected_players,
+        game_flow_statistics = {
+            pollution_statistics = {
+                input = game.pollution_statistics.input_counts,
+                output = game.pollution_statistics.output_counts,
+            },
+        },
+        force_flow_statistics = {}
+    }
+    for _, force in pairs(game.forces) do
+        local flow_statistics = {}
+        for _, statName in pairs(statistics) do
+            flow_statistics[statName] = {
+                input = force[statName].input_counts,
+                output = force[statName].output_counts,
+            }
+        end
+        stats.force_flow_statistics[force.name] = flow_statistics
+    end
+
+
+    rcon.print(game.table_to_json(stats))
+end
+
+return {
+    add_remote_interface = function()
+        remote.add_interface("statistics_exporter", { export = export })
+    end
+}