Telemetry handler that propagates OpenTelemetry context to LiveView processes.

This module attaches telemetry handlers for LiveView lifecycle events (mount, handle_params, handle_event) that run BEFORE opentelemetry_phoenix creates spans, ensuring the correct parent trace context is attached.

Why This Is Needed

When a browser makes an HTTP request with distributed tracing headers, the trace context is correctly extracted for the initial request. However, Phoenix LiveView spawns new BEAM processes for WebSocket connections that handle lifecycle callbacks.

opentelemetry_phoenix uses telemetry handlers to create spans for these events. If we don't inject the parent context BEFORE those handlers run, each LiveView span becomes a new root trace instead of being nested under the original HTTP request.

This module solves this by:

1. Using Sentry.Plug.LiveViewContext to store trace context in the session during the initial HTTP request
2. Attaching telemetry handlers with higher priority (registered first) than opentelemetry_phoenix
3. Extracting the context from the session and attaching it before opentelemetry_phoenix creates spans

Usage

Call setup/0 in your application's start function, BEFORE calling OpentelemetryPhoenix.setup/1:

def start(_type, _args) do
  # Set up Sentry's LiveView context propagation FIRST
  Sentry.OpenTelemetry.LiveViewPropagator.setup()

  # Then set up OpentelemetryPhoenix
  OpentelemetryPhoenix.setup(adapter: :bandit)

  children = [
    # ...
  ]

  Supervisor.start_link(children, strategy: :one_for_one)
end

Also add Sentry.Plug.LiveViewContext to your router pipeline:

pipeline :browser do
  plug :accepts, ["html"]
  plug :fetch_session
  # ... other plugs
  plug Sentry.Plug.LiveViewContext
end

Available since v12.0.0.