Convert the ValueObserver instruments to use the LastValue aggegration.

[jkwatson]

See this spec update: open-telemetry/opentelemetry-specification#984

Resolves #1780

[codecov]

Codecov Report

Merging #1689 into master will decrease coverage by 0.07%.
The diff coverage is 0.00%.

@@             Coverage Diff              @@
##             master    #1689      +/-   ##
============================================
- Coverage     85.34%   85.27%   -0.08%     
  Complexity     1352     1352              
============================================
  Files           165      165              
  Lines          5242     5242              
  Branches        539      539              
============================================
- Hits           4474     4470       -4     
- Misses          570      572       +2     
- Partials        198      200       +2     

Impacted Files	Coverage Δ	Complexity Δ
...ava/io/opentelemetry/sdk/metrics/ViewRegistry.java	63.63% <ø> (ø)	6.00 <0.00> (ø)
...o/opentelemetry/sdk/metrics/view/Aggregations.java	76.78% <0.00%> (ø)	4.00 <0.00> (ø)
...try/sdk/metrics/aggregator/LongMinMaxSumCount.java	95.91% <0.00%> (-4.09%)	6.00% <0.00%> (ø%)
...y/sdk/metrics/aggregator/DoubleMinMaxSumCount.java	95.91% <0.00%> (-4.09%)	6.00% <0.00%> (ø%)

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[bogdandrutu]

As mentioned in the Java maintainers meeting. This is partially fixing the issue because our current MetricData does not reflect the latest changes in OTLP and we should produce Gauge for this. Do we want to go ahead with this? That means we will have another breaking change soon when Gauge will be the correct result.

[jkwatson]

I think this is the right change for the current released version of OTLP, yes? I think it's fine to address the OTLP changes as a separate issue, once it gets released.

[bogdandrutu]

@jkwatson not really, https://github.com/open-telemetry/opentelemetry-java/blob/master/exporters/otlp/src/main/java/io/opentelemetry/exporters/otlp/MetricAdapter.java#L117

This will be exported as non-monotonic updown sum I believe which is wrong.

[jkwatson]

@jkwatson not really, https://github.com/open-telemetry/opentelemetry-java/blob/master/exporters/otlp/src/main/java/io/opentelemetry/exporters/otlp/MetricAdapter.java#L117

This will be exported as non-monotonic updown sum I believe which is wrong.

hmm. yeah, you're right. :(

[jkwatson]

@jkwatson not really, https://github.com/open-telemetry/opentelemetry-java/blob/master/exporters/otlp/src/main/java/io/opentelemetry/exporters/otlp/MetricAdapter.java#L117
This will be exported as non-monotonic updown sum I believe which is wrong.

hmm. yeah, you're right. :(

#1786 to track getting that fixed.