Clarify that exemplar reservoir default may change in a minor version

[jack-berg]

Related to #3756.

See this conversation for context.

[jack-berg]

cc @jsuereth / @reyang

[CodeBlanch]

I have a question about this:

Exemplar default reservoirs MAY change in a minor version bump. No guarantees are made on the shape or statistical properties of returned exemplars.

The spec has pretty clear defaults defined: https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/metrics/sdk.md#exemplar-defaults

@cijothomas Is recommending that in .NET we do a design that is this (more or less):

Exemplar defaults
The SDK will include three types of built-in exemplar reservoirs:

SimpleFixedSizeExemplarReservoir
AlignedHistogramBucketExemplarReservoir
NoopExemplarReservoir

By default:

All metrics will use the NoopExemplarReservoir by default. In order to capture Exemplars users MUST use the view API to select either SimpleFixedSizeExemplarReservoir, AlignedHistogramBucketExemplarReservoir, or a custom ExemplarReservoir.

This seems to be a big deviation from the spec switching Exemplars to be individually opt-in. My question is, was the change introduced on this PR intended to give SDKs such leeway?

[jack-berg]

No, the intent of this change was to allow us to later introduce new spec-defined reservoirs, and be able to change the default reservoir for a particular instrument type. In particular, there are likely improvements possible to the reservoir strategy for exponential bucket histograms.