feat(metrics): enhance WithFunctionName method to handle null or empty values and add corresponding unit tests

Author: hjgraca

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

@@ -86,7 +86,7 @@ public MetricsBuilder WithDefaultDimensions(Dictionary<string, string> defaultDi
     /// <returns></returns>
     public MetricsBuilder WithFunctionName(string functionName)
     {
-        _options.FunctionName = functionName;
+        _options.FunctionName = !string.IsNullOrWhiteSpace(functionName) ? functionName : null;
         return this;
     }
 

libraries/tests/AWS.Lambda.Powertools.Metrics.Tests/MetricsTests.cs

@@ -321,4 +321,55 @@ public void Service_Should_Return_Null_When_Not_Set()
         // Assert
         Assert.Null(result);
     }
+    
+    [Fact]
+    public void WithFunctionName_Should_Set_FunctionName_In_Options()
+    {
+        // Arrange
+        var builder = new MetricsBuilder();
+        var expectedFunctionName = "TestFunction";
+
+        // Act
+        var result = builder.WithFunctionName(expectedFunctionName);
+        var metrics = result.Build();
+
+        // Assert
+        Assert.Equal(expectedFunctionName, metrics.Options.FunctionName);
+        Assert.Same(builder, result);
+    }
+
+    [Theory]
+    [InlineData(null)]
+    [InlineData("")]
+    [InlineData(" ")]
+    public void WithFunctionName_Should_Allow_NullOrEmpty_FunctionName(string functionName)
+    {
+        // Arrange
+        var builder = new MetricsBuilder();
+
+        // Act
+        var result = builder.WithFunctionName(functionName);
+        var metrics = result.Build();
+
+        // Assert
+        // Assert
+        Assert.Null(metrics.Options.FunctionName); // All invalid values should result in null
+        Assert.Same(builder, result);
+    }
+
+    [Fact]
+    public void Build_Should_Preserve_FunctionName_When_Set_Through_Builder()
+    {
+        // Arrange
+        var builder = new MetricsBuilder()
+            .WithNamespace("TestNamespace")
+            .WithService("TestService")
+            .WithFunctionName("TestFunction");
+
+        // Act
+        var metrics = builder.Build();
+
+        // Assert
+        Assert.Equal("TestFunction", metrics.Options.FunctionName);
+    }
 }