Allow multiple exemplars on counters and histograms

[dashpole]

Fixes prometheus/OpenMetrics#311

Remove the restriction on having a single exemplar on counters. This matches PRW 2.0, OpenTelemetry, and makes exemplars more consistent between types.

While reading through exemplar language, I noticed the MUST requirement for histogram-bucket-aligned exemplars on fixed-bucket histograms. This relaxes (to SHOULD) the alignment of exemplars with histogram buckets. This is done because:

• Consistency between types (counters, histograms, and native histograms support multiple exemplars without restrictions.
• Reduces the amount of validation required by clients.
• Allows native histograms and classic histograms to share exemplar sampling in Prometheus clients (when storing both types internally), without confining exemplars to the classic buckets.

[krajorama]

Please update the ABNF (I can help)

[krajorama]

LGTM