[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, 20 insertions, 2 deletions

diff --git a/library/cpp/logger/stream.h b/library/cpp/logger/stream.h
index feb240afcb..a5eca9060e 100644
--- a/library/cpp/logger/stream.h
+++ b/library/cpp/logger/stream.h
@@ -2,11 +2,14 @@
 
 #include "backend.h"
 
+#include <util/generic/string.h>
+
+
 class IOutputStream;
 
-class TStreamLogBackend: public TLogBackend {
+class TStreamLogBackend : public TLogBackend {
 public:
-    TStreamLogBackend(IOutputStream* slave);
+    explicit TStreamLogBackend(IOutputStream* slave);
     ~TStreamLogBackend() override;
 
     void WriteData(const TLogRecord& rec) override;
@@ -15,3 +18,18 @@ public:
 private:
     IOutputStream* Slave_;
 };
+
+class TStreamWithContextLogBackend : public TLogBackend {
+private:
+    static constexpr TStringBuf DELIMITER = "; ";
+
+public:
+    explicit TStreamWithContextLogBackend(IOutputStream* slave);
+    ~TStreamWithContextLogBackend() override;
+
+    void WriteData(const TLogRecord& rec) override;
+    void ReopenLog() override;
+
+private:
+    IOutputStream* Slave_;
+};