Fix incorrect instance lookups in plugins

Fix mistaken lookups missing the instance part, or trying to get the id
from the removed instance.id property.

docs/writing-plugins.md

@@ -226,7 +226,7 @@ plugin you listen for an event named ipc-channel_name in order to get
 data sent by `send_json`.  For example in the plugin code:
 
     async init() {
-        this.server.on('ipc-my_plugin_foo', content =>
+        this.instance.server.on('ipc-my_plugin_foo', content =>
             this.handleFoo(content).catch(err => console.log(
                 "Error handling foo:", foo
             ))
@@ -478,7 +478,7 @@ accordingly, for example:
     );
 
     // Somewhere in the instance plugin code
-    barMetric.labels(String(this.config.id)).set(someValue);
+    barMetric.labels(String(this.instance.config.get("instance.id"))).set(someValue);
 
 Metrics are automatically registered to the default registry, and this
 default registry is automatically polled by the master server on slaves.

plugins/statistics_exporter/instance.js

@@ -30,7 +30,7 @@ class InstancePlugin extends plugin.BaseInstancePlugin {
 			throw new Error(`Error parsing statistics JSON: ${err.message}, content "${string}"`);
 		}
 
-		let instanceId = this.instance.config.id;
+		let instanceId = this.instance.config.get("instance.id");
 		instanceGameTicksTotal.labels(String(instanceId)).set(stats.game_tick);
 		instancePlayerCount.labels(String(instanceId)).set(stats.player_count);
 

test/mock.js

@@ -72,7 +72,10 @@ class MockInstance extends link.Link {
 		this.server = new MockServer();
 		this.name = "test";
 		this.config = {
-			id: 7357,
+			get: (name) => {
+				if (name === "instance.id") { return 7357 }
+				throw Error("Not implemented");
+			}
 		}
 	}
 }