Bleh `kraken`, fix another ref error in fill block

Clearly, the linter didn't help us here.. but, just pass the
`brokerd` time for now in the `.broker_time` field; we can't get it from
the fill-case incremental updates in the `openOrders` sub. Add some
notes about this and how we might approach for backends with this
limitation.

piker/brokers/kraken/broker.py

@@ -1024,7 +1024,7 @@ async def handle_order_updates(
                                 # TODO: maybe capture more msg data
                                 # i.e fees?
                                 broker_details={'name': 'kraken'} | order_msg,
-                                broker_time=broker_time
+                                broker_time=time.time(),
                             )
                             await ems_stream.send(fill_msg)
 

piker/ui/order_mode.py

@@ -985,7 +985,19 @@ async def process_trade_msg(
                 pointing='up' if action == 'buy' else 'down',
 
                 # TODO: put the actual exchange timestamp
-                arrow_index=get_index(details['broker_time']),
+                arrow_index=get_index(
+                    # TODO: note currently the ``kraken`` openOrders sub
+                    # doesn't deliver their engine timestamp as part of
+                    # it's schema, so this value is **not** from them
+                    # (see our backend code). We should probably either
+                    # include all provider-engine timestamps in the
+                    # summary 'closed' status msg and/or figure out
+                    # a way to indicate what is a `brokerd` stamp versus
+                    # a true backend one? This will require finagling
+                    # with how each backend tracks/summarizes time
+                    # stamps for the downstream API.
+                    details['broker_time']
+                ),
             )
 
             # TODO: how should we look this up?