OM 2.0: Consider specifying the keys used for trace id and span id

[dashpole]

Problem Statement

We provide trace correlation as an example use-case for exemplars.

A common use case are IDs of program traces.

The standard for propagating trace identifiers is the w3c Trace Context spec: https://www.w3.org/TR/trace-context-2/.

It would be helpful to provide guidance about how that trace identifier should be added to exemplars. For example, if prometheus clients want to provide any plugins or libraries to make it easier to add exemplars from OpenTelemetry or any other tracing library, it would help provide direction for how trace correlation should be done.

Proposed solution

Align with https://opentelemetry.io/docs/specs/otel/compatibility/prometheus_and_openmetrics/#exemplars-1, but only reference the w3c spec.

Proposed text:

When an exemplar references a Trace Context, it SHOULD use the trace_id key for the trace-id, and the span_id key for the parent-id.

Alternatives considered

Leave the spec as-is.