[SDK] PeriodicExportingMetricReader: future is never set, blocks until timeout

[tristan-lanfrey]

PeriodicExportingMetricReader::CollectAndExportOnce() future is never set and CollectAndExportOnce() probably blocks until timeout.

opentelemetry-cpp/sdk/src/metrics/export/periodic_exporting_metric_reader.cc

Line 106 in 9e062b5

std::promise<void> sender;

    std::promise<void> sender;
    auto receiver = sender.get_future();

nothing ever sets receiver's value. suggestion is to pass sender into the thread and have the thread call sender.set_value() when finished, in order to unblock the below receiver.wait_for() as soon as possible.

I think the saving grace here is the break when the future eventually times out.