Collect and Export metric data before PeriodicMetricReader shutdown

[sfc-gh-sili]

Reporting the exact same issue as #1858

It seems that the issue was previously fixed in #1860, but the latest code does not have do another round of export on PeriodicMetricReader on Shutdown anymore.

[lalitb]

@sfc-gh-sili This was changed as part of #2064. You need to now call Meterprovider::Forceflush() before MeterProvider::Shutdown() for one last collect cycle. The reason is

• It offers greater flexibility for applications that wish to shut down promptly, as the collection cycle can be time-consuming when dealing with a large number of different metrics, and they may not want to delay.
• It aligns with the shutdown process for traces and logs. Applications need to invoke flush for exporting pending spans/logs before shutdown.
• The specifications do not require or mention this.

Closing this based on the information above, but please don't hesitate to reopen it if you find this valid issue.