Reset cached target info cursor when OpenMetricsRequested is false

Author: hez2010

src/OpenTelemetry.Exporter.Prometheus.HttpListener/Internal/PrometheusCollectionManager.cs

@@ -213,6 +213,10 @@ private ExportResult OnCollect(Batch<Metric> metrics)
                     }
                 }
             }
+            else
+            {
+                this.targetInfoBufferLength = -1;
+            }
 
             foreach (var metric in metrics)
             {