Fix @tag_args being off-by-one (ahead)

changelog.d/13452.misc

@@ -0,0 +1 @@
+Fix `@tag_args` being off-by-one with the arguments when tagging a span (tracing).

synapse/logging/opentracing.py

@@ -909,8 +909,9 @@ def tag_args(func: Callable[P, R]) -> Callable[P, R]:
     @wraps(func)
     def _tag_args_inner(*args: P.args, **kwargs: P.kwargs) -> R:
         argspec = inspect.getfullargspec(func)
-        for i, arg in enumerate(argspec.args[1:]):
-            set_tag("ARG_" + arg, str(args[i]))  # type: ignore[index]
+        # We use `[1:]` to skip the `self` object reference
+        for i, arg in enumerate(args[1:]):
+            set_tag("ARG_" + argspec.args[i + 1], str(arg))  # type: ignore[index]
         set_tag("args", str(args[len(argspec.args) :]))  # type: ignore[index]
         set_tag("kwargs", str(kwargs))
         return func(*args, **kwargs)