Set label value to timestamp over 0, to keep track of recent history …

…of adapter queue and active adapters

vllm/engine/metrics.py

@@ -441,7 +441,7 @@ def _log_histogram(self, histogram, data: Union[List[int],
             histogram.labels(**self.labels).observe(datum)
 
     def _log_gauge_string(self, gauge, data: Dict[str, str]) -> None:
-        gauge.labels(**data).set(1)
+        gauge.labels(**data).set_to_current_time()
 
     def _log_prometheus(self, stats: Stats) -> None:
         # System state data