fix: remove timestamp rounding in TerseJsonFormatter

The round(record.created, 2) call limits timestamps generated by
TerseJsonFormatter to 10ms precision. This can cause log ordering issues
in log aggregators like Loki when multiple events occur within the same
10ms window.

The rounding was introduced in the original structured logging PR and,
to my knowledge, has no technical benefit.

synapse/logging/_terse_json.py

@@ -89,7 +89,7 @@ class TerseJsonFormatter(JsonFormatter):
             "log": record.getMessage(),
             "namespace": record.name,
             "level": record.levelname,
-            "time": round(record.created, 2),
+            "time": record.created,
         }
 
         return self._format(record, event)