Fix metric expressions with @return (#10178)

When compiling the metric expression using @return synthetic variable,
the type of @return was not propagated correctly and the generic
Object type was used instead which prevent to call length() when this
 is a String.
Fix also reporting errors as ProbeSatus by returning Error status
when instrumenting.

Author: jpbempel

dd-java-agent/agent-debugger/src/main/java/com/datadog/debugger/instrumentation/MetricInstrumenter.java

@@ -82,6 +82,7 @@ public class MetricInstrumenter extends Instrumenter {
   private final MetricProbe metricProbe;
   private int durationStartVar = -1;
   private LabelNode durationStartLabel;
+  private InstrumentationResult.Status status = InstrumentationResult.Status.INSTALLED;
 
   public MetricInstrumenter(
       MetricProbe metricProbe,
@@ -116,7 +117,13 @@ public InstrumentationResult.Status instrument() {
         throw new IllegalArgumentException(
             "Invalid evaluateAt attribute: " + definition.getEvaluateAt());
     }
-    return InstrumentationResult.Status.INSTALLED;
+    return status;
+  }
+
+  @Override
+  protected void reportError(String message) {
+    super.reportError(message);
+    status = InstrumentationResult.Status.ERROR;
   }
 
   private void addFinallyHandler(LabelNode endLabel) {
@@ -185,7 +192,7 @@ protected InsnList getBeforeReturnInsnList(AbstractInsnNode node) {
       case Opcodes.ARETURN:
         storeOpCode = Opcodes.ASTORE;
         loadOpCode = Opcodes.ALOAD;
-        returnType = OBJECT_TYPE;
+        returnType = Type.getReturnType(this.methodNode.desc);
         break;
       default:
         throw new UnsupportedOperationException("Unsupported opcode: " + node.getOpcode());

dd-java-agent/agent-debugger/src/test/java/com/datadog/debugger/agent/MetricProbesInstrumentationTest.java

@@ -399,6 +399,25 @@ public void methodSyntheticReturnGaugeMetric() throws IOException, URISyntaxExce
     assertArrayEquals(new String[] {METRIC_PROBEID_TAG}, listener.lastTags);
   }
 
+  @Test
+  public void methodSyntheticReturnLenGaugeMetric() throws IOException, URISyntaxException {
+    final String CLASS_NAME = "com.datadog.debugger.CapturedSnapshot32";
+    String METRIC_NAME = "syn_gauge";
+    MetricProbe metricProbe =
+        createMetricBuilder(METRIC_ID, METRIC_NAME, GAUGE)
+            .where(CLASS_NAME, "processArg", "(String)")
+            .valueScript(new ValueScript(DSL.len(DSL.ref("@return")), "len(@return)"))
+            .evaluateAt(MethodLocation.EXIT)
+            .build();
+    MetricForwarderListener listener = installMetricProbes(metricProbe);
+    Class<?> testClass = compileAndLoadClass(CLASS_NAME);
+    int result = Reflect.on(testClass).call("main", "foobar").get();
+    assertEquals(42, result);
+    assertTrue(listener.gauges.containsKey(METRIC_NAME));
+    assertEquals(6, listener.gauges.get(METRIC_NAME).longValue());
+    assertArrayEquals(new String[] {METRIC_PROBEID_TAG}, listener.lastTags);
+  }
+
   @Test
   public void methodSyntheticReturnInvalidType() throws IOException, URISyntaxException {
     final String CLASS_NAME = "CapturedSnapshot06";

dd-java-agent/agent-debugger/src/test/resources/MyResource.class

@@ -0,0 +1,15 @@
+package com.datadog.debugger;
+
+public class CapturedSnapshot32 {
+  public static int main(String arg) throws Exception {
+    arg = processArg(arg);
+    return 42;
+  }
+
+  public static String processArg(String arg) {
+    if (arg.startsWith("-")) {
+      arg = arg.substring(1);
+    }
+    return arg;
+  }
+}