HITL: server-side helpers to measure client latency

[eundersander]

Motivation and Context

Add ClientMessageManager.set_server_keyframe_id and RemoteGuiInput.pop_recent_server_keyframe_id to help the server measure client latency.

This feature will require follow-up work in the Unity client. stub_client.html provides a reference implementation.

In the example code below, roughly speaking, this is the latency between something happening in a sim_step on the server and the client's reaction (as GuiInput) being available in a later sim_step.

Usage: consider a server AppState that maintains a frame counter. It uses ClientMessageManager.set_server_keyframe_id on every frame. The client receives and renders a frame with serverKeyframeID=k. It should include recentServerKeyframeId=k in the next clientState it sends to the server. (See stub_client.html for an example.). In the server AppState, it can check RemoteGuiInput.pop_recent_server_keyframe_id() and compare it to its current frame counter.

Example usage in an AppState:

def __init__(...):
    self._frame_counter = 0

def sim_update(...):
    ...
    if self._app_service.remote_gui_input:
        recent_server_keyframe_id = self._app_service.remote_gui_input.pop_recent_server_keyframe_id()
        if recent_server_keyframe_id:
            print(f"client frame latency: {self._frame_counter - recent_server_keyframe_id}")

    if self._app_service.client_message_manager:
        self._app_service.client_message_manager.set_server_keyframe_id(self._frame_counter)
    self._frame_counter += 1

How Has This Been Tested

Local testing with rearrange and pick_throw_vr AppStates. With stub HTML client. Regression testing with Unity in-editor client (although it doesn't yet implement the required client-side functionality for this feature).

Types of changes

[HITL]

Checklist

• My code follows the code style of this project.
• I have updated the documentation if required.
• I have read the CONTRIBUTING document.
• I have completed my CLA (see CONTRIBUTING)
• I have added tests to cover my changes if required.

[eundersander]

Some comments.

[eundersander]

It seems like a good general rule that consolidating messages simply means updating to the latest message in the latest incremental keyframe. Let's beware a future situation where a client must absolutely receive an entire sequence of messages (not just the latest one).

[eundersander]

As part of this PR, the stub HTML client now sends client states at 10 Hz.

[0mdc]

LGTM!

[0mdc]

👍