Add :cumulative aggregation_temporality to metrics aggregators

[kaylareopelle]

Details on cumulative temporality in the specs:

• https://opentelemetry.io/docs/specs/otel/metrics/data-model/#temporality
• https://opentelemetry.io/docs/specs/otel/metrics/sdk/#metricreader

Relates to #1554

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[wsmoak]

I arrived here from the code comment saying the default for a sum aggregation should be cumulative:

opentelemetry-ruby/metrics_sdk/lib/opentelemetry/sdk/metrics/aggregation/sum.rb

Lines 16 to 20 in deac860

	def initialize(aggregation_temporality: ENV.fetch('OTEL_EXPORTER_OTLP_METRICS_TEMPORALITY_PREFERENCE', :delta), monotonic: false)
	# TODO: the default should be :cumulative, see issue #1555
	@aggregation_temporality = aggregation_temporality.to_sym
	@monotonic = monotonic
	end

The issue description isn't clear to me about what needs to happen.

[kaylareopelle]

Hi @wsmoak, thanks for checking in on this one. #1769 will resolve this issue and update the defaults. I'm wrapping up reviews on it today.

When I wrote this comment, I didn't realize the cumulative temporality would already be applied through an else clause in the collect methods for various aggregations. Here's an example that highlights the conditional branch for cumulative logic on an Explicit Bucket Histogram aggregation

opentelemetry-ruby/metrics_sdk/lib/opentelemetry/sdk/metrics/aggregation/explicit_bucket_histogram.rb

Lines 43 to 51 in e71e1bc

	else
	# Update timestamps and take a snapshot.
	data_points.values.map! do |hdp|
	hdp.start_time_unix_nano ||= start_time # Start time of a data point is from the first observation.
	hdp.time_unix_nano = end_time
	hdp = hdp.dup
	hdp.bucket_counts = hdp.bucket_counts.dup
	hdp
	end