Add scary warning that default recording_id = authkey (#3799)

Rendered:

![image](https://github.com/rerun-io/rerun/assets/2910679/645c216b-488a-4ef5-9627-e476bb29e8e7)

- Fixes #3798

rerun_py/rerun_sdk/rerun/__init__.py

@@ -3,6 +3,7 @@
 import functools
 import random
 from typing import Any, Callable, TypeVar, cast
+from uuid import UUID
 
 import numpy as np
 
@@ -247,10 +248,11 @@ def _init_recording_stream() -> None:
 _init_recording_stream()
 
 
+# TODO(#3793): defaulting recording_id to authkey should be opt-in
 def init(
     application_id: str,
     *,
-    recording_id: str | None = None,
+    recording_id: str | UUID | None = None,
     spawn: bool = False,
     init_logging: bool = True,
     default_enabled: bool = True,
@@ -266,6 +268,26 @@ def init(
 
     For more advanced use cases, e.g. multiple recordings setups, see [`rerun.new_recording`][].
 
+    !!! Warning
+        If you don't specify a `recording_id`, it will default to a random value that is generated once
+        at the start of the process.
+        That value will be kept around for the whole lifetime of the process, and even inherited by all
+        its subprocesses, if any.
+
+        This makes it trivial to log data to the same recording in a multiprocess setup, but it also means
+        that the following code will _not_ create two distinct recordings:
+        ```
+        rr.init("my_app")
+        rr.init("my_app")
+        ```
+
+        To create distinct recordings from the same process, specify distinct recording IDs:
+        ```
+        from uuid import uuid4
+        rr.init("my_app", recording_id=uuid4())
+        rr.init("my_app", recording_id=uuid4())
+        ```
+
     Parameters
     ----------
     application_id : str
@@ -322,6 +344,9 @@ def init(
     # via `_register_on_fork` but it's worth being conservative.
     cleanup_if_forked_child()
 
+    if recording_id is not None:
+        recording_id = str(recording_id)
+
     if init_logging:
         new_recording(
             application_id=application_id,
@@ -348,10 +373,11 @@ def init(
         _spawn()
 
 
+# TODO(#3793): defaulting recording_id to authkey should be opt-in
 def new_recording(
     *,
     application_id: str,
-    recording_id: str | None = None,
+    recording_id: str | UUID | None = None,
     make_default: bool = False,
     make_thread_default: bool = False,
     spawn: bool = False,
@@ -362,6 +388,26 @@ def new_recording(
 
     If you only need a single global recording, [`rerun.init`][] might be simpler.
 
+    !!! Warning
+        If you don't specify a `recording_id`, it will default to a random value that is generated once
+        at the start of the process.
+        That value will be kept around for the whole lifetime of the process, and even inherited by all
+        its subprocesses, if any.
+
+        This makes it trivial to log data to the same recording in a multiprocess setup, but it also means
+        that the following code will _not_ create two distinct recordings:
+        ```
+        rr.init("my_app")
+        rr.init("my_app")
+        ```
+
+        To create distinct recordings from the same process, specify distinct recording IDs:
+        ```
+        from uuid import uuid4
+        rec = rr.new_recording(application_id="test", recording_id=uuid4())
+        rec = rr.new_recording(application_id="test", recording_id=uuid4())
+        ```
+
     Parameters
     ----------
     application_id : str
@@ -432,6 +478,9 @@ def new_recording(
     except Exception:
         pass
 
+    if recording_id is not None:
+        recording_id = str(recording_id)
+
     recording = RecordingStream(
         bindings.new_recording(
             application_id=application_id,