[Frontend] correctly record prefill and decode time metrics (#10853)

Signed-off-by: Tomer Asida <[email protected]>

Author: tomeras91

vllm/engine/metrics.py

@@ -599,9 +599,9 @@ def _log_prometheus(self, stats: Stats) -> None:
                             stats.time_queue_requests)
         self._log_histogram(self.metrics.histogram_inference_time_request,
                             stats.time_inference_requests)
-        self._log_histogram(self.metrics.histogram_decode_time_request,
-                            stats.time_prefill_requests)
         self._log_histogram(self.metrics.histogram_prefill_time_request,
+                            stats.time_prefill_requests)
+        self._log_histogram(self.metrics.histogram_decode_time_request,
                             stats.time_decode_requests)
         self._log_histogram(self.metrics.histogram_time_in_queue_request,
                             stats.time_in_queue_requests)