Add a discarding MetricsObserver

This is useful for test code where we don't care about the metrics
gathered.

projects/core/src/main/java/dan200/computercraft/core/metrics/DiscardingMetricsObserver.java

@@ -0,0 +1,23 @@
+// SPDX-FileCopyrightText: 2023 The CC: Tweaked Developers
+//
+// SPDX-License-Identifier: MPL-2.0
+
+package dan200.computercraft.core.metrics;
+
+/**
+ * A {@link MetricsObserver} implementation which discards all metrics it receives.
+ */
+final class DiscardingMetricsObserver implements MetricsObserver {
+    static final MetricsObserver INSTANCE = new DiscardingMetricsObserver();
+
+    private DiscardingMetricsObserver() {
+    }
+
+    @Override
+    public void observe(Metric.Counter counter) {
+    }
+
+    @Override
+    public void observe(Metric.Event event, long value) {
+    }
+}

projects/core/src/main/java/dan200/computercraft/core/metrics/MetricsObserver.java

@@ -31,4 +31,13 @@ public interface MetricsObserver {
      * @param value The value corresponding to this event.
      */
     void observe(Metric.Event event, long value);
+
+    /**
+     * Get a {@link MetricsObserver} which discards all metrics.
+     *
+     * @return An observer which discards all metrics.
+     */
+    static MetricsObserver discard() {
+        return DiscardingMetricsObserver.INSTANCE;
+    }
 }

projects/core/src/testFixtures/java/dan200/computercraft/test/core/computer/BasicEnvironment.java

@@ -12,7 +12,6 @@
 import dan200.computercraft.core.filesystem.FileMount;
 import dan200.computercraft.core.filesystem.JarMount;
 import dan200.computercraft.core.filesystem.MemoryMount;
-import dan200.computercraft.core.metrics.Metric;
 import dan200.computercraft.core.metrics.MetricsObserver;
 
 import java.io.File;
@@ -27,7 +26,7 @@
  * A basic implementation of {@link ComputerEnvironment} and {@link GlobalEnvironment}, suitable for a context which
  * will only run a single computer.
  */
-public class BasicEnvironment implements ComputerEnvironment, GlobalEnvironment, MetricsObserver {
+public class BasicEnvironment implements ComputerEnvironment, GlobalEnvironment {
     private final WritableMount mount;
 
     public BasicEnvironment() {
@@ -55,7 +54,7 @@ public double getTimeOfDay() {
 
     @Override
     public MetricsObserver getMetrics() {
-        return this;
+        return MetricsObserver.discard();
     }
 
     @Override
@@ -124,12 +123,4 @@ private static File getContainingFile(Class<?> klass) {
             return new File(url.getPath());
         }
     }
-
-    @Override
-    public void observe(Metric.Counter counter) {
-    }
-
-    @Override
-    public void observe(Metric.Event event, long value) {
-    }
 }

projects/web/src/main/java/cc/tweaked/web/EmulatedComputer.java

@@ -19,7 +19,6 @@
 import dan200.computercraft.core.computer.ComputerEnvironment;
 import dan200.computercraft.core.computer.ComputerSide;
 import dan200.computercraft.core.filesystem.MemoryMount;
-import dan200.computercraft.core.metrics.Metric;
 import dan200.computercraft.core.metrics.MetricsObserver;
 import dan200.computercraft.core.terminal.Terminal;
 import org.slf4j.Logger;
@@ -37,7 +36,7 @@
  * <p>
  * This is exposed to Javascript via the {@link ComputerHandle} interface.
  */
-class EmulatedComputer implements ComputerEnvironment, ComputerHandle, MetricsObserver {
+class EmulatedComputer implements ComputerEnvironment, ComputerHandle {
     private static final Logger LOG = LoggerFactory.getLogger(EmulatedComputer.class);
 
     private static final ComputerSide[] SIDES = ComputerSide.values();
@@ -124,15 +123,7 @@ public WritableMount createRootMount() {
 
     @Override
     public MetricsObserver getMetrics() {
-        return this;
-    }
-
-    @Override
-    public void observe(Metric.Counter counter) {
-    }
-
-    @Override
-    public void observe(Metric.Event event, long value) {
+        return MetricsObserver.discard();
     }
 
     @Override