Support explicit exporting of metrics in Lambda/Serverless environment.

[arszen123]

Is your feature request related to a problem?

I am trying to export metrics from AWS Lambda with the PeriodicExportingMetricReader, by calling forceFlush at the end of the handler function. However, after checking out the implementation, I realized that the metrics are not exported here.

Describe the solution you'd like

I would like to modify the PeriodicExportingMetricReader#forceFlush method to export the metrics here as well, before calling forceFlush on the exporter. Also, considering to add exporting to the shutdown function.

Describe alternatives you've considered

An alternative solution could be to add a new on-demand metric reader, that only exports metrics when forceFlush or shutdown is called. This would make sense if someone wants to export metrics at different breakpoints, for example at the end of the Lamda function.

Additional context

Example code:

const meterProvider = new MeterProvider();
const reader = new PeriodicExportingMetricReader({ /** options **/});

meterProvider.addMetricReader(reader);

exports.handler = async () => {
  // do some work

  // force metrics exporting
  await meterProvider.forceFlush();

  return;
}

[legendecas]

I agree it makes sense for PeriodicExportingMetricReader to expose a method to collect and export metrics.

[github-actions]

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