Make Delta vs Cumulative based purely on Exporter

[cijothomas]

Changes

1. Makes Delta vs Cumulative decision tied to exporter. Its an exporter level setting, so all metrics either becomes delta or cumulative.
   Once Views are in, this'd require rework anyway, as views likely would allow per-instrument delta vs cumulative setting.

2. Modified ConsoleExporter to pick between delta and cumulative, defaulting to delta

Cumulative Console Exporter

using var meterProvider = Sdk.CreateMeterProviderBuilder()
                .AddSource("TestMeter")
                .AddConsoleExporter(options => options.IsDelta = false)
                .Build();

Delta Console Exporter

using var meterProvider = Sdk.CreateMeterProviderBuilder()
                .AddSource("TestMeter")
                .AddConsoleExporter(options => options.IsDelta = true) //default
                .Build();

[codecov]

Codecov Report

Merging #2142 (73df7b2) into metrics (63c28f0) will decrease coverage by 0.02%.
The diff coverage is 0.00%.

@@             Coverage Diff             @@
##           metrics    #2142      +/-   ##
===========================================
- Coverage    78.27%   78.24%   -0.03%     
===========================================
  Files          212      212              
  Lines         6632     6634       +2     
===========================================
  Hits          5191     5191              
- Misses        1441     1443       +2     

Impacted Files	Coverage Δ
src/OpenTelemetry/Metrics/AggregatorStore.cs	0.00% <0.00%> (ø)
src/OpenTelemetry/Metrics/MeterProviderSdk.cs	0.00% <0.00%> (ø)
...Metrics/MetricAggregators/GaugeMetricAggregator.cs	0.00% <ø> (ø)
...ics/MetricAggregators/HistogramMetricAggregator.cs	0.00% <0.00%> (ø)
...y/Metrics/MetricAggregators/SumMetricAggregator.cs	0.00% <0.00%> (ø)
...trics/MetricAggregators/SummaryMetricAggregator.cs	0.00% <ø> (ø)
...elemetry/Metrics/Processors/PullMetricProcessor.cs	0.00% <0.00%> (ø)
...elemetry/Metrics/Processors/PushMetricProcessor.cs	0.00% <0.00%> (ø)

[alanwest]

I think a boolean is fine, but just pondering another idea... what if we had an enumeration for AggregationTemporality and this setting was instead

public AggregationTemporality AggregationTemporality { get; set; } = AggregationTemporality.Delta

Also, this brings up an interesting question of defaults. What's the sense from the spec regarding defaults for SDK provided exporters?

[reyang]

+1 with AggregationTemporality

[reyang]

I think defaults to delta makes more sense (for console, delta generates less output). A good topic for the SDK spec :)

[cijothomas]

Agree. making it in next PR :)

[alanwest]

LGTM

[reyang]

LGTM.