Update Metrics constructor to not return and set all values.

Add tests

Author: hjgraca

libraries/src/AWS.Lambda.Powertools.Metrics/Metrics.cs

@@ -65,15 +65,15 @@ public class Metrics : IMetrics
     internal Metrics(IPowertoolsConfigurations powertoolsConfigurations, string nameSpace = null, string service = null,
         bool raiseOnEmptyMetrics = false, bool captureColdStartEnabled = false)
     {
-        if (_instance != null) return;
+        _instance ??= this;
 
-        _instance = this;
         _powertoolsConfigurations = powertoolsConfigurations;
         _raiseOnEmptyMetrics = raiseOnEmptyMetrics;
         _captureColdStartEnabled = captureColdStartEnabled;
         _context = InitializeContext(nameSpace, service, null);
 
         _powertoolsConfigurations.SetExecutionEnvironment(this);
+        
     }
 
     /// <summary>

libraries/tests/AWS.Lambda.Powertools.Metrics.Tests/Handlers/ExceptionFunctionHandler.cs

@@ -7,13 +7,28 @@ namespace AWS.Lambda.Powertools.Metrics.Tests.Handlers;
 public class ExceptionFunctionHandler
 {
     [Metrics(Namespace = "ns", Service = "svc")]
-    public async Task<string> Handle(string input)
+    public Task<string> Handle(string input)
     {
         ThisThrows();
+        return Task.FromResult(input.ToUpper(CultureInfo.InvariantCulture));
+    }
+    
+    [Metrics(Namespace = "ns", Service = "svc")]
+    public Task<string> HandleDecoratorOutsideHandler(string input)
+    {
+        MethodDecorated();
+        
+        Metrics.AddMetric($"Metric Name", 1, MetricUnit.Count);
 
-        await Task.Delay(1);
+        return Task.FromResult(input.ToUpper(CultureInfo.InvariantCulture));
+    }
 
-        return input.ToUpper(CultureInfo.InvariantCulture);
+    [Metrics(Namespace = "ns", Service = "svc")]
+    private void MethodDecorated()
+    {
+        // NOOP
+        Metrics.AddMetric($"Metric Name", 1, MetricUnit.Count);
+        Metrics.AddMetric($"Metric Name Decorated", 1, MetricUnit.Count);
     }
 
     private void ThisThrows()

libraries/tests/AWS.Lambda.Powertools.Metrics.Tests/Handlers/ExceptionFunctionHandlerTests.cs

@@ -22,4 +22,21 @@ public async Task Stack_Trace_Included_When_Decorator_Present()
         Assert.StartsWith("at AWS.Lambda.Powertools.Metrics.Tests.Handlers.ExceptionFunctionHandler.ThisThrows()", tracedException.StackTrace?.TrimStart());
 
     }
+    
+    [Fact]
+    public async Task Decorator_In_Non_Handler_Method_Does_Not_Throw_Exception()
+    {
+        // Arrange
+        Metrics.ResetForTest();
+        var handler = new ExceptionFunctionHandler();
+
+        // Act
+        Task Handle() => handler.HandleDecoratorOutsideHandler("whatever");
+        
+        // Assert
+        var tracedException = await Record.ExceptionAsync(Handle);
+        Assert.Null(tracedException);
+        //Assert.StartsWith("at AWS.Lambda.Powertools.Metrics.Tests.Handlers.ExceptionFunctionHandler.ThisThrows()", tracedException.StackTrace?.TrimStart());
+
+    }
 }