Allow selection of exponential vs. explicit bucket histogram for OTLP metric exporters via environment variable

[evantorrie]

Problem Statement

The Go SDK supports configuration of OTLP exporters' default aggregation temporality (via OTEL_EXPORTER_OTLP_METRICS_TEMPORALITY_PREFERENCE), but does not appear to implement configuration of the default aggregation via OTEL_EXPORTER_OTLP_METRICS_DEFAULT_HISTOGRAM_AGGREGATION.

Proposed Solution

Implement selection of exponential histogram as the default histogram aggregation for OTLP metric exporters via the environment variable OTEL_EXPORTER_OTLP_METRICS_DEFAULT_HISTOGRAM_AGGREGATION.

Alternatives

When created via code, the New() function can always be called with WithAggregationSelector() to pass an arbitrary function as the aggregation selector, but the specification says that OTLP exporter aggregation for histograms should be able to be configured without needing to write code.

Prior Art

Seems that solution should be very similar to that used for the temporality preference