fix(trace): respect trace start_at time as is

Prior to this fix, the start_at provided when creating the trace
is overriden if it's earlier than the current system time.

This is unnecessary, and leads to test cases being flaky.

apps/emqx/src/emqx_trace/emqx_trace.erl

@@ -651,8 +651,7 @@ to_trace(#{type := Type}, _Rec) ->
 to_trace(#{payload_encode := PayloadEncode} = Trace, Rec) ->
     to_trace(maps:remove(payload_encode, Trace), Rec#?TRACE{payload_encode = PayloadEncode});
 to_trace(#{start_at := StartAt} = Trace, Rec) ->
-    {ok, Sec} = to_system_second(StartAt),
-    to_trace(maps:remove(start_at, Trace), Rec#?TRACE{start_at = Sec});
+    to_trace(maps:remove(start_at, Trace), Rec#?TRACE{start_at = StartAt});
 to_trace(#{end_at := EndAt} = Trace, Rec) ->
     Now = now_second(),
     case to_system_second(EndAt) of

apps/emqx/src/emqx_trace/emqx_trace_dl.erl

@@ -53,6 +53,7 @@ update(Name, Enable) ->
 insert_new_trace(Trace) ->
     case mnesia:read(?TRACE, Trace#?TRACE.name) of
         [] ->
+            %% allow one new trace for each filter in the same second
             #?TRACE{start_at = StartAt, type = Type, filter = Filter} = Trace,
             Match = #?TRACE{_ = '_', start_at = StartAt, type = Type, filter = Filter},
             case mnesia:match_object(?TRACE, Match, read) of