Exemplar: Refactor to be under metrics.data.

[songy23]

Blocked by #1779.

[codecov-io]

Codecov Report

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The diff coverage is 80%.

@@            Coverage Diff            @@
##             master    #1791   +/-   ##
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  Coverage          ?   83.81%           
  Complexity        ?     2029           
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  Files             ?      294           
  Lines             ?     9227           
  Branches          ?      891           
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  Hits              ?     7734           
  Misses            ?     1177           
  Partials          ?      316

Impacted Files	Coverage Δ	Complexity Δ
...ava/io/opencensus/metrics/export/Distribution.java	96.96% <ø> (ø)	7 <0> (?)
...trib/exemplar/util/AttachmentValueSpanContext.java	100% <ø> (ø)	3 <0> (?)
.../src/main/java/io/opencensus/stats/MeasureMap.java	40% <ø> (ø)	2 <0> (?)
...io/opencensus/implcore/stats/MeasureToViewMap.java	94.93% <ø> (ø)	28 <0> (?)
.../opencensus/implcore/stats/MeasureMapInternal.java	90.32% <ø> (ø)	4 <0> (?)
...java/io/opencensus/implcore/stats/MetricUtils.java	83.72% <ø> (ø)	8 <0> (?)
...ain/java/io/opencensus/common/AttachmentValue.java	100% <ø> (ø)	1 <0> (?)
...pencensus/contrib/exemplar/util/ExemplarUtils.java	100% <ø> (ø)	3 <0> (?)
...a/io/opencensus/implcore/stats/IntervalBucket.java	100% <ø> (ø)	11 <0> (?)
...main/java/io/opencensus/stats/AggregationData.java	97.43% <ø> (ø)	0 <0> (?)
... and 5 more

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[songy23]

Rebased and updated. The reasons for making this change:

1. By its nature Exemplar is a correlation between multiple signals (histogram and trace in our case). IMO it doesn't belong to an existing signal. We already made Exemplar an independent package in Go.
2. Exemplar and its Attachments are required at both recording time and exporting time. For recording we have stats package and for exporting we have metrics. We don't want stats and metrics APIs to have dependency on each other to avoid circular dependency. Therefore previously we have two identical implementations of Exemplar: stats.AggregationData.Exemplar and metrics.export.Distribution.Exemplar. This doesn't work if we were to use one consistent Exemplar for both recording and exporting.

Thus I proposed to extract Exemplar to common package so that both stats and metrics packages can depend on it. Note this is a breaking change to those who use the current Exemplar APIs.

[bogdandrutu]

I think we are moving to the model where stats is just for recording and not for exporting (currently all our exporters are now using Metric no more ViewData). Probably we should remove the ViewData, hence we will remove the AggregationData.

So I think we should work to remove the ViewData and AggregationData, so that will remove the duplicated code.

[songy23]

So I think we should work to remove the ViewData and AggregationData, so that will remove the duplicated code.

But still we need AttachmentValue in both recording APIs and exporting APIs:

statsRecorder.newMeasureMap().put(m, v).putAttachment(key, attachmentValue).record();
...
metricProducer.getMetrics().get(0)...getDistribution().getExemplars().get(0).getAttachments();

so the only viable option I saw here is still to extract it to a common artifact.

[bogdandrutu]

For the common types that we need during recording as well as exporting I suggest we do what Go does for metrics and add a "data" sub-package. stats, metrics and metrics.exporter can depend on metrics.data.

[songy23]

I suggest we do what Go does for metrics and add a "data" sub-package. stats, metrics and metrics.exporter can depend on metrics.data.

OK that makes sense to me. I'll post a separate PR for adding that package first.

[songy23]

4eea6a2 moved Exemplar and AttachmentValue to metrics.data so that both stats and metrics packages can depend on it. PTAL.