fix: filter session count sums by created_at (#6529)

Fixes the session totals being waaaaay too high!

coderd/database/queries/workspaceagentstats.sql

@@ -70,7 +70,7 @@ WITH agent_stats AS (
 		coalesce(SUM(session_count_reconnecting_pty), 0)::bigint AS session_count_reconnecting_pty
 	 FROM (
 		SELECT *, ROW_NUMBER() OVER(PARTITION BY agent_id ORDER BY created_at DESC) AS rn
-		FROM workspace_agent_stats
+		FROM workspace_agent_stats WHERE created_at > $1
 	) AS a WHERE a.rn = 1
 )
 SELECT * FROM agent_stats, latest_agent_stats;