Add LogDog / annotation protobuf support.

recipe_engine/arguments.proto

Index: recipe_engine/arguments.proto
diff --git a/recipe_engine/arguments.proto b/recipe_engine/arguments.proto
index e639466d9979435e435d00e5c715339c5f36bc44..579cd41d13cdc12d79d5613ead632669d82949c7 100644
--- a/recipe_engine/arguments.proto
+++ b/recipe_engine/arguments.proto
@@ -64,4 +64,23 @@ message Arguments {
   }
   // Annotation control flags.
   AnnotationFlags annotation_flags = 2;
+
+  // LogDog flags.
+  //
+  // If the "streamserver_uri" is provided, recipe output will be forwarded
+  // through LogDog streams using Milo's Annotation Protobuf instead of
+  // STDOUT/STDERR and classic @@@annotations@@@.
+  message LogDogFlags {
+    // The LogDog streamserver URI.
+    string streamserver_uri = 1;
+
+    // The log stream base name. If provided, generated stream names will be
+    // prefixed with "<name_base>/". This must be a valid LogDog stream name.
+    string name_base = 2;
+
+    // If true, tee output through STDOUT/STDERR using inline @@@annotation@@@
+    // markers in addition to LogDog streaming.
+    bool tee = 3;
+  }
+  LogDogFlags logdog = 3;
 }