feat: use traceparent header from incoming shape requests to set parent span

.changeset/odd-walls-walk.md

@@ -0,0 +1,5 @@
+---
+"@core/sync-service": patch
+---
+
+use traceparent header from incoming shape requests to set parent span

packages/sync-service/lib/electric/plug/router.ex

@@ -10,6 +10,7 @@ defmodule Electric.Plug.Router do
   plug Plug.Head
   plug :match
   plug Electric.Plug.LabelProcessPlug
+  plug Electric.Plug.TraceContextPlug
   plug Plug.Telemetry, event_prefix: [:electric, :routing]
   plug Plug.Logger
   plug :put_cors_headers

packages/sync-service/lib/electric/plug/trace_context_plug.ex

@@ -0,0 +1,58 @@
+defmodule Electric.Plug.TraceContextPlug do
+  @moduledoc """
+  A plug that extracts trace context from incoming HTTP headers and sets it as the parent span.
+  """
+  @behaviour Plug
+
+  require Logger
+
+  def init(opts), do: opts
+
+  def call(conn, _opts) do
+    case extract_trace_context(conn) do
+      {:ok, span_ctx} ->
+        :otel_tracer.set_current_span(span_ctx)
+        conn
+
+      :error ->
+        conn
+    end
+  end
+
+  # Extract trace context using the OpenTelemetry propagator
+  defp extract_trace_context(conn) do
+    # Get all headers as a list of {key, value} tuples
+    headers =
+      conn
+      |> Plug.Conn.get_req_header("traceparent")
+      |> Enum.map(fn value -> {"traceparent", value} end)
+
+    # Create a new context and extract the trace context from headers
+    ctx =
+      :otel_propagator_trace_context.extract(
+        :otel_ctx.new(),
+        headers,
+        :undefined,
+        &header_getter/2,
+        %{}
+      )
+
+    # Get the span context from the extracted context
+    case :otel_tracer.current_span_ctx(ctx) do
+      :undefined -> :error
+      span_ctx -> {:ok, span_ctx}
+    end
+  end
+
+  # Header getter function for the propagator
+  # Note: The key is passed first, carrier second by the propagator
+  defp header_getter(key, carrier) when is_list(carrier) do
+    case Enum.find(carrier, fn {k, _v} -> String.downcase(k) == String.downcase(key) end) do
+      {_key, value} -> value
+      nil -> []
+    end
+  end
+
+  # Fallback clause for when carrier is not a list
+  defp header_getter(_key, _carrier), do: []
+end