Fix default value of OtlpHttpMetricExporterOptions::aggregation_temporality. (open-telemetry#1601)

Author: owent

exporters/otlp/include/opentelemetry/exporters/otlp/otlp_http_metric_exporter.h

@@ -55,7 +55,7 @@ struct OtlpHttpMetricExporterOptions
 
   // Preferred Aggregation Temporality
   sdk::metrics::AggregationTemporality aggregation_temporality =
-      sdk::metrics::AggregationTemporality::kCumulative;
+      sdk::metrics::AggregationTemporality::kDelta;
 
 #  ifdef ENABLE_ASYNC_EXPORT
   // Concurrent requests