Rename "replication lag" to "retained WAL size"

The thing being measured here is the size of the WAL that Electric's
replication slot retains, so I think the new name describes it better.

This change also restructures the code a bit to make the dependency on
the DB connection pool crystal clear.

packages/sync-service/lib/electric/connection/manager.ex

@@ -146,8 +146,8 @@ defmodule Electric.Connection.Manager do
     GenServer.cast(server, {:pg_info_looked_up, pg_info})
   end
 
-  def query_replication_lag(server) do
-    GenServer.call(server, :query_replication_lag)
+  def report_retained_wal_size(server) do
+    GenServer.call(server, :report_retained_wal_size)
   end
 
   @impl true
@@ -231,8 +231,12 @@ defmodule Electric.Connection.Manager do
     {:reply, :ok, %{state | drop_slot_requested: true}}
   end
 
-  def handle_call(:query_replication_lag, _from, state) do
-    report_replication_lag(state)
+  def handle_call(:report_retained_wal_size, _from, state) do
+    if state.monitoring_started? do
+      slot_name = Keyword.fetch!(state.replication_opts, :slot_name)
+      query_and_report_retained_wal_size(state.pool_pid, slot_name)
+    end
+
     {:reply, :ok, state}
   end
 
@@ -650,20 +654,22 @@ defmodule Electric.Connection.Manager do
     end
   end
 
-  defp report_replication_lag(%{monitoring_started?: false}), do: :ok
-
-  defp report_replication_lag(%{pool_pid: pool} = state) do
-    slot_name = Keyword.fetch!(state.replication_opts, :slot_name)
-
-    query =
-      "SELECT pg_wal_lsn_diff(pg_current_wal_lsn(), confirmed_flush_lsn) AS replication_lag_size FROM pg_replication_slots WHERE slot_name = $1;"
+  defp query_and_report_retained_wal_size(pool, slot_name) do
+    query = """
+    SELECT
+      pg_wal_lsn_diff(pg_current_wal_lsn(), confirmed_flush_lsn)
+    FROM
+      pg_replication_slots
+    WHERE
+      slot_name = $1
+    """
 
     case Postgrex.query(pool, query, [slot_name]) do
-      {:ok, %Postgrex.Result{rows: [[lag]]}} ->
-        :telemetry.execute([:electric, :postgres, :replication], %{lag: lag})
+      {:ok, %Postgrex.Result{rows: [[wal_size]]}} ->
+        :telemetry.execute([:electric, :postgres, :replication], %{wal_size: wal_size})
 
       {:error, error} ->
-        Logger.warning("Failed to query replication lag\nError: #{inspect(error)}")
+        Logger.warning("Failed to query retained WAL size\nError: #{inspect(error)}")
     end
 
     :ok

packages/sync-service/lib/electric/telemetry.ex

@@ -93,7 +93,7 @@ defmodule Electric.Telemetry do
     [
       # A module, function and arguments to be invoked periodically.
       {__MODULE__, :uptime_event, []},
-      {Electric.Connection.Manager, :query_replication_lag,
+      {Electric.Connection.Manager, :report_retained_wal_size,
        [Electric.Connection.Manager.name(stack_id)]}
     ]
   end