Metrics: Specify the default aggregation for Measure instruments

[jmacd]

The current specification says that Measure instruments default to "MinMaxSumCount" aggregation, which outputs four values. The question is whether it typically makes more sense to reduce this to only the Sum and Count aggregations (i.e., two values).

See this comment: #430 (comment)

The rationale for making Min/Max/Sum and Count aggregations be the default is that these four values are all cheap-to-compute summary statistics. It was mentioned in today's SIG that these are the default behavior for a Prometheus Summary metric instrument, but I could not find evidence for this assertion.

[jmacd]

@bogdandrutu I believe your position was that Min and Max are not often used and will cause unnecessary reporting. Is that it?

I like to include Min and Max because it cheaply tells us something about the distribution of values.

[jmacd]

This will close when OTEP 98 is approved and merged into the specification.
Please review: open-telemetry/oteps#98