Remove the sdk/metric/aggregator/exact aggregator

[jmacd]

Problem Statement

This aggregator dates from very early in the prototype. It was used to demonstrate a Summary-like metric aggregation, including the raw data used to calculate precise quantiles. However, it is a misleading type and produces confusion, particularly because the OTLP exporter turns the type into raw gauge points, meaning it exposes a route from Histogram instruments to Gauge data.

Twice today I answered questions from confused users who used the simple NewWithExactDistribution() aggregator selector and got a bunch of gauge points.

Proposed Solution

Remove this code.

Alternatives

Users can implement this aggregator easily. One of the "near misses" for this code is to support passing raw data through the OTel API directly to another metrics library. This aggregator doesn't support that use-case very well.