feat(sync-service): populate num_bytes and subset_result span attributes

.changeset/populate-telemetry-columns.md

@@ -0,0 +1,5 @@
+---
+'@core/sync-service': patch
+---
+
+Populate previously-empty telemetry span attributes for shape requests: `num_bytes` on the root shape-get span and `electric.subqueries.subset_result.{bytes,rows,duration_µs}` on subset materialisation spans. This makes the corresponding Honeycomb columns queryable.

.gitignore

@@ -30,4 +30,5 @@ response.tmp
 .claude
 !website/.claude/commands
 !website/.claude/skills
-_artifacts
+_artifacts
+.agent-tasks

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

@@ -291,11 +291,13 @@ defmodule Electric.Plug.ServeShapePlug do
   # is the place to assign them because we keep this plug last in the "plug pipeline" defined
   # in this module.
   defp end_telemetry_span(%Conn{assigns: assigns} = conn, _ \\ nil) do
+    bytes_sent = assigns[:streaming_bytes_sent] || 0
+
     OpenTelemetry.execute(
       [:electric, :plug, :serve_shape],
       %{
         count: 1,
-        bytes: assigns[:streaming_bytes_sent] || 0,
+        bytes: bytes_sent,
         monotonic_time: System.monotonic_time(),
         duration: System.monotonic_time() - conn.private[:electric_telemetry_span][:start_time]
       },
@@ -308,6 +310,12 @@ defmodule Electric.Plug.ServeShapePlug do
       }
     )
 
+    # Expose the total response body size as a dedicated span attribute
+    # alongside the semantic `http.response_size` set by
+    # `Electric.Plug.Utils.common_open_telemetry_attrs/1`. This populates the
+    # `num_bytes` Honeycomb column for shape-get requests.
+    OpenTelemetry.add_span_attributes(%{num_bytes: bytes_sent})
+
     add_span_attrs_from_conn(conn)
     OpentelemetryTelemetry.end_telemetry_span(OpenTelemetry, %{})
     conn

packages/sync-service/lib/electric/shapes/partial_modes.ex

@@ -63,10 +63,12 @@ defmodule Electric.Shapes.PartialModes do
         {[row], {start_time, bytes + IO.iodata_length(row), rows + 1}}
       end,
       fn {start_time, bytes, rows} ->
+        duration = System.monotonic_time(:microsecond) - start_time
+
         OpenTelemetry.execute(
           [:electric, :subqueries, :subset_result],
           %{
-            duration: System.monotonic_time(:microsecond) - start_time,
+            duration: duration,
             bytes: bytes,
             count: 1,
             rows: rows
@@ -77,6 +79,15 @@ defmodule Electric.Shapes.PartialModes do
             "shape.root_table": shape.root_table
           }
         )
+
+        # Expose subset materialisation metrics as span attributes on the
+        # active shape_snapshot.query_fn child span so they become queryable
+        # Honeycomb columns. Mirrors the telemetry event measurements above.
+        OpenTelemetry.add_span_attributes(%{
+          "electric.subqueries.subset_result.bytes" => bytes,
+          "electric.subqueries.subset_result.rows" => rows,
+          "electric.subqueries.subset_result.duration_µs" => duration
+        })
       end
     )
   end