[PoC] Implement Metric Producer specification

[dashpole]

Specification: https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/metrics/sdk.md#metricproducer

It includes open-telemetry/opentelemetry-specification#3613, as producers are passed to readers as configuration, rather than via a registration method.

The tricky part was how to handle Resource (see open-telemetry/opentelemetry-specification#3636). This is tricky because getResource is part of MetricData, but we would rather the OpenCensus shim not need to understand resource. In this PoC, i've opted to split the MetricProducer interface into two interfaces: SdkMetricProducer, and MetricProducer.

• SdkMetricProducer can provide the resource to readers, and returns batches of metrics that already include Resource (MetricData)
• MetricProducer returns batches of metrics that do not include Resource (ScopeMetricData).

Alternatively, the produce() function on MetricProducer could accept Resource as a parameter, and return MetricData. That would introduce less of an API surface, but could be misused.

example usage:

SdkMeterProvider.builder().registerMetricReader(PeriodicMetricReader.builder(exporter).addMetricProducer(OpenCensusMetricProducer.create()).build())

TODOs:

• Unit tests
• Fix the prometheus unit test

@jack-berg @jsuereth

[codecov]

Codecov Report

Patch coverage: 57.54% and project coverage change: -0.70% ⚠️

Comparison is base (7ae32c9) 91.31% compared to head (39b347b) 90.61%.
Report is 2 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #5678      +/-   ##
============================================
- Coverage     91.31%   90.61%   -0.70%     
- Complexity     4978     4988      +10     
============================================
  Files           554      555       +1     
  Lines         14731    14855     +124     
  Branches       1376     1388      +12     
============================================
+ Hits          13451    13461      +10     
- Misses          884      969      +85     
- Partials        396      425      +29     

Files Changed	Coverage Δ
...ry/sdk/metrics/internal/export/MetricProducer.java	0.00% <ø> (-50.00%)	⬇️
...pentelemetry/sdk/metrics/data/ScopeMetricData.java	27.27% <27.27%> (ø)
...porter/prometheus/PrometheusHttpServerBuilder.java	59.09% <40.00%> (-40.91%)	⬇️
...lemetry/sdk/testing/assertj/ScopeMetricAssert.java	43.75% <43.75%> (ø)
...metry/sdk/metrics/export/PeriodicMetricReader.java	78.48% <46.15%> (-7.24%)	⬇️
...dk/metrics/export/PeriodicMetricReaderBuilder.java	86.95% <50.00%> (-13.05%)	⬇️
...etry/exporter/prometheus/PrometheusHttpServer.java	55.93% <53.33%> (-24.81%)	⬇️
...sdk/metrics/internal/export/SdkMetricProducer.java	77.77% <77.77%> (ø)
...y/opencensusshim/OpenTelemetryMetricsExporter.java	85.29% <87.50%> (+1.96%)	⬆️
...etrics/internal/data/ImmutableScopeMetricData.java	88.88% <88.88%> (ø)
... and 7 more

... and 1 file with indirect coverage changes

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