Fix METGEN004 error message: print return type in ErrorInvalidMethodReturnType

Author: eduherminio

src/Generators/Microsoft.Gen.Metrics/Parser.cs

@@ -574,7 +574,7 @@ private bool CheckMethodReturnType(IMethodSymbol methodSymbol)
             returnType.TypeKind != TypeKind.Error)
         {
             // Make sure return type is not from existing known type
-            Diag(DiagDescriptors.ErrorInvalidMethodReturnType, methodSymbol.ReturnType.GetLocation(), methodSymbol.Name);
+            Diag(DiagDescriptors.ErrorInvalidMethodReturnType, returnType.GetLocation(), returnType.ToDisplayString(SymbolDisplayFormat.MinimallyQualifiedFormat));
             return false;
         }
 

test/Generators/Microsoft.Gen.Metrics/Unit/ParserTests.cs

@@ -81,6 +81,7 @@ class CustomClass {{ }}
 
         _ = Assert.Single(d);
         Assert.Equal(DiagDescriptors.ErrorInvalidMethodReturnType.Id, d[0].Id);
+        Assert.Contains($"must not return '{returnType}'", d[0].GetMessage());
     }
 
     [Theory]