test(metrics): Improve testability of @FlushMetrics annotation

[phipag]

Summary

This PR allows customers to use a plain Lambda context mock when unit testing request handlers annotated with @FlushMetrics. This addresses an issue where an exception was raised due to a missing AWS Request ID when attempting to collect the cold start metric.

After merging this PR, the following unit test should pass (I also included such a test in the metrics module itself):

@ExtendWith(MockitoExtension.class)
class RequestHandlerTest {

    // For developer convenience, no exceptions should be thrown when using a plain Lambda Context mock
    @Mock
    Context lambdaContext;

    private final PrintStream standardOut = System.out;
    private ByteArrayOutputStream outputStreamCaptor;
    private final ObjectMapper objectMapper = new ObjectMapper();

    @BeforeEach
    void setUp() {
        outputStreamCaptor = new ByteArrayOutputStream();
        System.setOut(new PrintStream(outputStreamCaptor));
    }

    @AfterEach
    void tearDown() throws Exception {
        System.setOut(standardOut);
    }

    @Test
    void shouldCaptureMetricsFromAnnotatedHandler() throws Exception {
        // Given
        RequestHandler<Map<String, Object>, String> handler = new HandlerWithMetricsAnnotation();
        Map<String, Object> input = new HashMap<>();

        // When
        handler.handleRequest(input, lambdaContext);

        // Then
        String emfOutput = outputStreamCaptor.toString().trim();
        JsonNode rootNode = objectMapper.readTree(emfOutput);

        assertThat(rootNode.has("test-metric")).isTrue();
        assertThat(rootNode.get("test-metric").asDouble()).isEqualTo(100.0);
        assertThat(rootNode.get("_aws").get("CloudWatchMetrics").get(0).get("Namespace").asText())
                .isEqualTo("CustomNamespace");
        assertThat(rootNode.has("Service")).isTrue();
        assertThat(rootNode.get("Service").asText()).isEqualTo("CustomService");
    }

    static class HandlerWithMetricsAnnotation implements RequestHandler<Map<String, Object>, String> {
        @Override
        @FlushMetrics(namespace = "CustomNamespace", service = "CustomService")
        public String handleRequest(Map<String, Object> input, Context context) {
            Metrics metrics = MetricsFactory.getMetricsInstance();
            metrics.addMetric("test-metric", 100, MetricUnit.COUNT);
            return "OK";
        }
    }
}

Changes

Issue number: #2019

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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