update MetricsAdapter to include exemplars for counters (#949)

Author: angelodipaolo

Sources/Exporters/OpenTelemetryProtocolCommon/metric/MetricsAdapter.swift

@@ -228,6 +228,14 @@ public enum MetricsAdapter {
     pointData.exemplars.forEach {
       var protoExemplar = Opentelemetry_Proto_Metrics_V1_Exemplar()
       protoExemplar.timeUnixNano = $0.epochNanos
+		
+	  if let doubleExemplar = $0 as? DoubleExemplarData {
+		protoExemplar.value = .asDouble(doubleExemplar.value)
+	  }
+
+	  if let longExemplar = $0 as? LongExemplarData {
+		protoExemplar.value = .asInt(Int64(longExemplar.value))
+	  }
 
       $0.filteredAttributes.forEach {
         protoExemplar.filteredAttributes.append(CommonAdapter.toProtoAttribute(key: $0.key, attributeValue: $0.value))