Shutting down a MeterProvider with a asynchronous instrument can take a long time

[duncanpo]

Run the following code in a loop. Many iterations take only about 0.01 seconds to run but some can take a long time (~30 seconds).

function out = currentsecond()
% callback function for an observable gauge

dt = datetime("now");
s = second(dt);

out = opentelemetry.metrics.ObservableResult;
out = out.observe(s);

for i=1:10, 
   mp = opentelemetry.sdk.metrics.MeterProvider();  
   opentelemetry.metrics.Provider.setMeterProvider(mp);
   clear
   mp = opentelemetry.metrics.Provider.getMeterProvider();   
   m = getMeter(mp, "foo"); 
   g = createObservableGauge(m, @currentsecond, "bar");
   tic;   
   opentelemetry.sdk.common.Cleanup.shutdown(mp);   
   toc; 
end

In comparison, if the code is changed to create a synchronous instrument (e.g. counter) instead of an asynchronous one, then shutdown never takes a long time, always around 0.01 seconds.

This issue causes an asynchronous example test in test/texamples.m to be filtered out due to the time it takes to run the test.