OM 2.0: exemplars provided for complex types MUST have a timestamp

[krajorama]

Problem Statement

From prometheus/docs#2634 (comment). Depends on complex types being introduced via the same PR 2634.

In general it is not possible to handle all cases of sets of exemplars provided for a complex type (histogram/summary) if we do not require that the timestamp of the exemplar is present.

Proposed solution

Require that exemplars provided for complex types have a timestamp.

Alternatives considered

Keep the wording SHOULD, but note that implementations may drop such exemplars as it is not possible to handle them.

[dashpole]

I think we should also require timestamps on non-complex types, and support multiple exemplars on those types as well.

Rationale:

• PRW 2.0 supports multiple exemplars for all types
• OTel SDKs produce multiple exemplars for counters
• Consistency with complex types (simpler spec)

[dashpole]

I will open a separate issue to track multiple exemplars.