Clarify how to mock the Lambda context in unit tests for Metrics utility.

Author: phipag

docs/core/metrics.md

@@ -558,10 +558,11 @@ If you would like to suppress metrics output during your unit tests, you can use
 
 When unit testing your code, you can run assertions against the output generated by the `Metrics` Singleton. For the `EmfMetricsLogger`, you can assert the generated JSON blob following the [CloudWatch EMF specification](https://docs.aws.amazon.com/AmazonCloudWatch/latest/monitoring/CloudWatch_Embedded_Metric_Format.html) against your expected output.
 
-Consider the following example where we redirect the standard output to a custom `PrintStream`. We use the Jackson library to parse the EMF output into a `JsonNode` and run assertions against that.
+Consider the following example where we redirect the standard output to a custom `PrintStream`. We use the Jackson library to parse the EMF output into a `JsonNode` and run assertions against that. Note that it is important to mock the Lambda `Context` with realistic values so that the metrics have valid metadata and dimensions during unit tests.
 
-```java hl_lines="23 28 33 50-55"
+```java hl_lines="26-27 35 37-42 47 63-68"
 import static org.assertj.core.api.Assertions.assertThat;
+import static org.mockito.Mockito.when;
 
 import java.io.ByteArrayOutputStream;
 import java.io.PrintStream;
@@ -571,24 +572,37 @@ import java.util.Map;
 import org.junit.jupiter.api.AfterEach;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.ExtendWith;
+import org.mockito.Mock;
+import org.mockito.junit.jupiter.MockitoExtension;
 
 import com.amazonaws.services.lambda.runtime.Context;
 import com.amazonaws.services.lambda.runtime.RequestHandler;
 import com.fasterxml.jackson.databind.JsonNode;
 import com.fasterxml.jackson.databind.ObjectMapper;
 
 import software.amazon.lambda.powertools.metrics.model.MetricUnit;
-import software.amazon.lambda.powertools.metrics.testutils.TestContext;
 
+@ExtendWith(MockitoExtension.class)
 class MetricsTestExample {
 
+    @Mock
+    Context lambdaContext;
+
     private final PrintStream standardOut = System.out;
     private final ByteArrayOutputStream outputStreamCaptor = new ByteArrayOutputStream();
     private final ObjectMapper objectMapper = new ObjectMapper();
 
     @BeforeEach
     void setUp() {
         System.setOut(new PrintStream(outputStreamCaptor));
+
+        // Mock a minimal realistic Lambda context. These values are used in metadata and dimensions by the Metrics utility.
+        when(lambdaContext.getAwsRequestId()).thenReturn("test-request-id");
+        when(lambdaContext.getFunctionName()).thenReturn("test-function");
+        when(lambdaContext.getFunctionVersion()).thenReturn("test-version");
+        when(lambdaContext.getInvokedFunctionArn()).thenReturn("test-arn");
+        when(lambdaContext.getMemoryLimitInMB()).thenReturn(128);
     }
 
     @AfterEach
@@ -600,11 +614,10 @@ class MetricsTestExample {
     void shouldCaptureMetricsFromAnnotatedHandler() throws Exception {
         // Given
         RequestHandler<Map<String, Object>, String> handler = new HandlerWithMetricsAnnotation();
-        Context context = new TestContext();
         Map<String, Object> input = new HashMap<>();
 
         // When
-        handler.handleRequest(input, context);
+        handler.handleRequest(input, lambdaContext);
 
         // Then
         String emfOutput = outputStreamCaptor.toString().trim();