diff --git a/docs/core/metrics.md b/docs/core/metrics.md
index 31079a76e..61d4c38f0 100644
--- a/docs/core/metrics.md
+++ b/docs/core/metrics.md
@@ -140,7 +140,7 @@ For most use-cases, we recommend using Environment variables and only overwrite
             Type: AWS::Serverless::Function
             Properties:
             ...
-            Runtime: java8
+            Runtime: java11
             Environment:
                 Variables:
                     POWERTOOLS_SERVICE_NAME: payment
@@ -558,9 +558,24 @@ 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.
 
+Make sure to set a test metrics namespace and service name to run assertions against metrics. For example, by setting the following environment variables in your tests:
+
+```xml
+<plugin>
+    <groupId>org.apache.maven.plugins</groupId>
+    <artifactId>maven-surefire-plugin</artifactId>
+    <configuration>
+        <environmentVariables>
+            <POWERTOOLS_SERVICE_NAME>TestService</POWERTOOLS_SERVICE_NAME>
+            <POWERTOOLS_METRICS_NAMESPACE>TestNamespace</POWERTOOLS_METRICS_NAMESPACE>
+        </environmentVariables>
+    </configuration>
+</plugin>
+```
+
 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.
 
-```java hl_lines="23 28 33 50-55"
+```java hl_lines="35 40 56-72"
 import static org.assertj.core.api.Assertions.assertThat;
 
 import java.io.ByteArrayOutputStream;
@@ -571,6 +586,9 @@ 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;
@@ -578,21 +596,25 @@ 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 ByteArrayOutputStream outputStreamCaptor;
     private final ObjectMapper objectMapper = new ObjectMapper();
 
     @BeforeEach
     void setUp() {
+        outputStreamCaptor = new ByteArrayOutputStream();
         System.setOut(new PrintStream(outputStreamCaptor));
     }
 
     @AfterEach
-    void tearDown() {
+    void tearDown() throws Exception {
         System.setOut(standardOut);
     }
 
@@ -600,27 +622,37 @@ 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();
-        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");
+        String[] jsonLines = emfOutput.split("\n");
+
+        // First JSON object should be the cold start metric
+        JsonNode coldStartNode = objectMapper.readTree(jsonLines[0]);
+        assertThat(coldStartNode.has("ColdStart")).isTrue();
+        assertThat(coldStartNode.get("ColdStart").asDouble()).isEqualTo(1.0);
+        assertThat(coldStartNode.get("_aws").get("CloudWatchMetrics").get(0).get("Namespace").asText())
+                .isEqualTo("TestNamespace");
+        assertThat(coldStartNode.has("Service")).isTrue();
+        assertThat(coldStartNode.get("Service").asText()).isEqualTo("TestService");
+
+        // Second JSON object should be the regular metric
+        JsonNode regularNode = objectMapper.readTree(jsonLines[1]);
+        assertThat(regularNode.has("test-metric")).isTrue();
+        assertThat(regularNode.get("test-metric").asDouble()).isEqualTo(100.0);
+        assertThat(regularNode.get("_aws").get("CloudWatchMetrics").get(0).get("Namespace").asText())
+                .isEqualTo("TestNamespace");
+        assertThat(regularNode.has("Service")).isTrue();
+        assertThat(regularNode.get("Service").asText()).isEqualTo("TestService");
     }
 
     static class HandlerWithMetricsAnnotation implements RequestHandler<Map<String, Object>, String> {
         @Override
-        @FlushMetrics(namespace = "CustomNamespace", service = "CustomService")
+        @FlushMetrics(captureColdStart = true)
         public String handleRequest(Map<String, Object> input, Context context) {
             Metrics metrics = MetricsFactory.getMetricsInstance();
             metrics.addMetric("test-metric", 100, MetricUnit.COUNT);
diff --git a/docs/core/tracing.md b/docs/core/tracing.md
index 883f8db86..8129d45ba 100644
--- a/docs/core/tracing.md
+++ b/docs/core/tracing.md
@@ -105,7 +105,7 @@ Before your use this utility, your AWS Lambda function [must have permissions](h
             Type: AWS::Serverless::Function
             Properties:
             ...
-            Runtime: java8
+            Runtime: java11
     
             Tracing: Active
             Environment:
@@ -191,7 +191,7 @@ different supported `captureMode` to record response, exception or both.
             Type: AWS::Serverless::Function
             Properties:
             ...
-            Runtime: java8
+            Runtime: java11
     
             Tracing: Active
             Environment:
diff --git a/powertools-metrics/pom.xml b/powertools-metrics/pom.xml
index 4bccab505..b90506c25 100644
--- a/powertools-metrics/pom.xml
+++ b/powertools-metrics/pom.xml
@@ -105,6 +105,11 @@
             <artifactId>assertj-core</artifactId>
             <scope>test</scope>
         </dependency>
+        <dependency>
+            <groupId>org.mockito</groupId>
+            <artifactId>mockito-junit-jupiter</artifactId>
+            <scope>test</scope>
+        </dependency>
         <dependency>
             <groupId>software.amazon.lambda</groupId>
             <artifactId>powertools-common</artifactId>
@@ -135,6 +140,13 @@
         </profile>
         <profile>
             <id>generate-graalvm-files</id>
+            <dependencies>
+                <dependency>
+                    <groupId>org.mockito</groupId>
+                    <artifactId>mockito-subclass</artifactId>
+                    <scope>test</scope>
+                </dependency>
+            </dependencies>
             <build>
                 <plugins>
                     <plugin>
@@ -154,6 +166,13 @@
         </profile>
         <profile>
             <id>graalvm-native</id>
+            <dependencies>
+                <dependency>
+                    <groupId>org.mockito</groupId>
+                    <artifactId>mockito-subclass</artifactId>
+                    <scope>test</scope>
+                </dependency>
+            </dependencies>
             <build>
                 <plugins>
                     <plugin>
diff --git a/powertools-metrics/src/main/java/software/amazon/lambda/powertools/metrics/internal/EmfMetricsLogger.java b/powertools-metrics/src/main/java/software/amazon/lambda/powertools/metrics/internal/EmfMetricsLogger.java
index 2f6f9e689..c9651585f 100644
--- a/powertools-metrics/src/main/java/software/amazon/lambda/powertools/metrics/internal/EmfMetricsLogger.java
+++ b/powertools-metrics/src/main/java/software/amazon/lambda/powertools/metrics/internal/EmfMetricsLogger.java
@@ -204,7 +204,7 @@ public void captureColdStartMetric(Context context,
             }
 
             // Add request ID from context if available
-            if (context != null) {
+            if (context != null && context.getAwsRequestId() != null) {
                 coldStartLogger.putProperty(REQUEST_ID_PROPERTY, context.getAwsRequestId());
             }
 
diff --git a/powertools-metrics/src/main/java/software/amazon/lambda/powertools/metrics/internal/LambdaMetricsAspect.java b/powertools-metrics/src/main/java/software/amazon/lambda/powertools/metrics/internal/LambdaMetricsAspect.java
index b214d7c52..32824e24f 100644
--- a/powertools-metrics/src/main/java/software/amazon/lambda/powertools/metrics/internal/LambdaMetricsAspect.java
+++ b/powertools-metrics/src/main/java/software/amazon/lambda/powertools/metrics/internal/LambdaMetricsAspect.java
@@ -113,7 +113,10 @@ private void captureColdStartMetricIfEnabled(Context extractedContext, FlushMetr
         }
 
         Metrics metricsInstance = MetricsFactory.getMetricsInstance();
-        metricsInstance.addMetadata(REQUEST_ID_PROPERTY, extractedContext.getAwsRequestId());
+        // This can be null e.g. during unit tests when mocking the Lambda context
+        if (extractedContext.getAwsRequestId() != null) {
+            metricsInstance.addMetadata(REQUEST_ID_PROPERTY, extractedContext.getAwsRequestId());
+        }
 
         // Only capture cold start metrics if enabled on annotation
         if (metrics.captureColdStart()) {
@@ -133,8 +136,9 @@ private void captureColdStartMetricIfEnabled(Context extractedContext, FlushMetr
             }
 
             // Add function name
-            coldStartDimensions.addDimension("FunctionName",
-                    funcName != null ? funcName : extractedContext.getFunctionName());
+            if (funcName != null) {
+                coldStartDimensions.addDimension("FunctionName", funcName);
+            }
 
             metricsInstance.captureColdStartMetric(extractedContext, coldStartDimensions);
         }
diff --git a/powertools-metrics/src/test/java/software/amazon/lambda/powertools/metrics/RequestHandlerTest.java b/powertools-metrics/src/test/java/software/amazon/lambda/powertools/metrics/RequestHandlerTest.java
new file mode 100644
index 000000000..d94a6bbe8
--- /dev/null
+++ b/powertools-metrics/src/test/java/software/amazon/lambda/powertools/metrics/RequestHandlerTest.java
@@ -0,0 +1,126 @@
+package software.amazon.lambda.powertools.metrics;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import java.io.ByteArrayOutputStream;
+import java.io.PrintStream;
+import java.lang.reflect.Method;
+import java.util.HashMap;
+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.junitpioneer.jupiter.SetEnvironmentVariable;
+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.common.internal.LambdaHandlerProcessor;
+import software.amazon.lambda.powertools.metrics.model.MetricUnit;
+
+@ExtendWith(MockitoExtension.class)
+class RequestHandlerTest {
+
+    // For developer convenience, no exceptions should be thrown when using a plain Lambda Context mock
+    @Mock
+    Context lambdaContext;
+
+    private static final PrintStream STDOUT = System.out;
+    private ByteArrayOutputStream outputStreamCaptor;
+    private final ObjectMapper objectMapper = new ObjectMapper();
+
+    @BeforeEach
+    void setUp() throws Exception {
+        outputStreamCaptor = new ByteArrayOutputStream();
+        System.setOut(new PrintStream(outputStreamCaptor));
+
+        // Reset LambdaHandlerProcessor's SERVICE_NAME
+        Method resetServiceName = LambdaHandlerProcessor.class.getDeclaredMethod("resetServiceName");
+        resetServiceName.setAccessible(true);
+        resetServiceName.invoke(null);
+
+        // Reset IS_COLD_START
+        java.lang.reflect.Field coldStartField = LambdaHandlerProcessor.class.getDeclaredField("IS_COLD_START");
+        coldStartField.setAccessible(true);
+        coldStartField.set(null, null);
+    }
+
+    @AfterEach
+    void tearDown() throws Exception {
+        System.setOut(STDOUT);
+
+        // Reset the singleton state between tests
+        java.lang.reflect.Field field = MetricsFactory.class.getDeclaredField("metrics");
+        field.setAccessible(true);
+        field.set(null, null);
+
+        field = MetricsFactory.class.getDeclaredField("provider");
+        field.setAccessible(true);
+        field.set(null, new software.amazon.lambda.powertools.metrics.provider.EmfMetricsProvider());
+    }
+
+    @Test
+    @SetEnvironmentVariable(key = "POWERTOOLS_METRICS_NAMESPACE", value = "TestNamespace")
+    @SetEnvironmentVariable(key = "POWERTOOLS_SERVICE_NAME", value = "TestService")
+    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();
+        String[] jsonLines = emfOutput.split("\n");
+
+        // First JSON object should be the cold start metric
+        JsonNode coldStartNode = objectMapper.readTree(jsonLines[0]);
+        assertThat(coldStartNode.has("ColdStart")).isTrue();
+        assertThat(coldStartNode.get("ColdStart").asDouble()).isEqualTo(1.0);
+        assertThat(coldStartNode.get("_aws").get("CloudWatchMetrics").get(0).get("Namespace").asText())
+                .isEqualTo("TestNamespace");
+        assertThat(coldStartNode.has("Service")).isTrue();
+        assertThat(coldStartNode.get("Service").asText()).isEqualTo("TestService");
+
+        // Second JSON object should be the regular metric
+        JsonNode regularNode = objectMapper.readTree(jsonLines[1]);
+        assertThat(regularNode.has("test-metric")).isTrue();
+        assertThat(regularNode.get("test-metric").asDouble()).isEqualTo(100.0);
+        assertThat(regularNode.get("_aws").get("CloudWatchMetrics").get(0).get("Namespace").asText())
+                .isEqualTo("TestNamespace");
+        assertThat(regularNode.has("Service")).isTrue();
+        assertThat(regularNode.get("Service").asText()).isEqualTo("TestService");
+    }
+
+    @SetEnvironmentVariable(key = "POWERTOOLS_METRICS_DISABLED", value = "true")
+    @Test
+    void shouldNotCaptureMetricsWhenDisabled() {
+        // 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();
+        assertThat(emfOutput).isEmpty();
+    }
+
+    static class HandlerWithMetricsAnnotation implements RequestHandler<Map<String, Object>, String> {
+        @Override
+        @FlushMetrics(captureColdStart = true)
+        public String handleRequest(Map<String, Object> input, Context context) {
+            Metrics metrics = MetricsFactory.getMetricsInstance();
+            metrics.addMetric("test-metric", 100, MetricUnit.COUNT);
+            return "OK";
+        }
+    }
+}