Fix PrometheusExporter reporting duplicate samples (#863)

Author: CharlieTLe

Sources/Exporters/Prometheus/PrometheusExporter.swift

@@ -35,14 +35,13 @@ public class PrometheusExporter: MetricExporter {
 
   public func export(metrics: [MetricData]) -> ExportResult {
     metricsLock.withLockVoid {
-      self.metrics.append(contentsOf: metrics)
+      self.metrics = metrics
     }
     return .success
   }
 
-  public func getAndClearMetrics() -> [MetricData] {
+  public func getMetrics() -> [MetricData] {
     defer {
-      metrics = [MetricData]()
       metricsLock.unlock()
     }
     metricsLock.lock()

Sources/Exporters/Prometheus/PrometheusExporterExtensions.swift

@@ -23,7 +23,7 @@ public enum PrometheusExporterExtensions {
 
   public static func writeMetricsCollection(exporter: PrometheusExporter) -> String {
     var output = ""
-    let metrics = exporter.getAndClearMetrics()
+    let metrics = exporter.getMetrics()
     let now = String(Int64((Date().timeIntervalSince1970 * 1000.0).rounded()))
 
     metrics.forEach { metric in