Implement aggregators for metrics

[lzchen]

Currently, only the aggregator for counter metrics has been implemented.
Implement the corresponding aggregators for the rest of the metrics types.

https://github.com/open-telemetry/opentelemetry-specification/blob/2b75442b05fd968f197422dc18124338a955f3a2/specification/sdk-metric.md#aggregator-implementations

[mauriciovasquezbernal]

I think we need to better define the scope of this issue. The link on the description is a document that is not part of the specification yet, so I don't know if we should base the decision of what aggregators to implement to that. Currently we have Counter, MinMaxSumCount and MinMaxSumCount (observer) aggregators. Which ones are we missing for the beta release?

[MikeGoldsmith]

Observer should have a "last value" aggregator.
OTEP-80
OTEL-Spec-412.

[mauriciovasquezbernal]

@MikeGoldsmith I made a typo, currently the Observer has a MinMaxSumCountLast aggregator. I remember there were some discussion about simplifying it, do you have any info of the final decision on that discussion?

[MikeGoldsmith]

@mauriciovasquezbernal - Metrics Spec just says LastValue aggregation 👍

[krnowak]

In golang, the simple selector for observer basically chooses an "measure"-like aggregator. See open-telemetry/opentelemetry-go#474 (comment)

So yeah, that aggregations part in metrics spec seems a bit conflicting with standard impls of instruments in metrics specs.

[lzchen]

@toumorokoshi @c24t
I think this issue can be closed. We have the default aggregators for each metric type and I believe that is sufficient enough for the beta release.