[library/cpp/logger] Support meta flags with TLogElement

Support opportunity to use TMetaFlags with TLogElement.

**Use example:**
`Log().With("Property", "Value").With("Code", "Code") << LogMessage`

Such interface allows to not parse required by TLogBackend  properties from raw string.

1 files changed, 16 insertions, 0 deletions

diff --git a/library/cpp/logger/element_ut.cpp b/library/cpp/logger/element_ut.cpp
index 32edc52dfb0..86303973d8f 100644
--- a/library/cpp/logger/element_ut.cpp
+++ b/library/cpp/logger/element_ut.cpp
@@ -9,13 +9,16 @@
 
 #include <library/cpp/testing/unittest/registar.h>
 
+
 class TLogElementTest: public TTestBase {
     UNIT_TEST_SUITE(TLogElementTest);
     UNIT_TEST(TestMoveCtor);
+    UNIT_TEST(TestWith);
     UNIT_TEST_SUITE_END();
 
 public:
     void TestMoveCtor();
+    void TestWith();
 };
 
 UNIT_TEST_SUITE_REGISTRATION(TLogElementTest);
@@ -37,3 +40,16 @@ void TLogElementTest::TestMoveCtor() {
     dst.Destroy();
     UNIT_ASSERT(output.Str() == message);
 }
+
+void TLogElementTest::TestWith() {
+    TStringStream output;
+    TLog log(MakeHolder<TStreamWithContextLogBackend>(&output));
+
+    THolder<TLogElement> src = MakeHolder<TLogElement>(&log);
+
+    TString message = "Hello, World!";
+    (*src).With("Foo", "Bar").With("Foo", "Baz") << message;
+
+    src.Destroy();
+    UNIT_ASSERT(output.Str() == "Hello, World!; Foo=Bar; Foo=Baz; ");
+}