[Metrics] ASync Instruments not fully implemented.

[marcalff]

Greetings,

I am a bit confused about the development status for async instruments in Metrics ...

There are no remaining issues left listed in the "Metrics SDK - Alpha release" milestone, great.

There are 5 unrelated open items in "Metrics SDK - Beta Release", and issue #1338 is about a more complex use case (observing N metrics at once), so this gives the impression that a basic Observable counter is already implemented.

If this is not supported yet, feel free to close this issue as a duplicate.
If supported, then it seems some bits are missing in the implementation:

Using #undef ENABLE_METRICS_PREVIEW

The method Meter::CreateLongObservableCounter() is available,
and builds a LongObservableCounter object, ok.

The constructors for:

• LongObservableCounter
• opentelemetry::metrics::ObservableCounter<long>
• Asynchronous<long>
• opentelemetry::metrics::AsynchronousInstrument

keep the instrument name + callback + description + unit,
but never build an instrument descriptor with it.

In the code path of Meter::CreateLongObservableCounter(),
the counter returned is not registered with any AsyncStorage,
and nothing keeps a reference to all the observable counters created,
so the ::Collect methods will never invoke callbacks on async instruments.

For synchronous instruments, methods like Meter::CreateLongCounter() build an InstrumentDescriptor object and call RegisterMetricStorage(), so I would expect some similar code, but found none.

Note that the class AsyncMetricStorage exists, but is never used in the production code (there is one use only, but from a unit test).

While testing, the callback function given to Meter::CreateLongObservableCounter() is never called,
and async instruments are not exported (tested with the Prometheus exporter)

What am I missing (or not understanding) ?

Regards.

[lalitb]

Thanks, will look into this.