feat(om2): classic histogram and summary as complex types (#2679)

* Classic histogram and summary as complex types

Ref: prometheus/OpenMetrics#283

Signed-off-by: György Krajcsovits <gyorgy.krajcsovits@grafana.com>

* Update docs/specs/om/open_metrics_spec_2_0.md

Co-authored-by: Bartlomiej Plotka <bwplotka@gmail.com>
Signed-off-by: George Krajcsovits <krajorama@users.noreply.github.com>

* fix summary quantile order and other omissions

Say exemplars, not exemplar.

Signed-off-by: György Krajcsovits <gyorgy.krajcsovits@grafana.com>

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Signed-off-by: György Krajcsovits <gyorgy.krajcsovits@grafana.com>
Signed-off-by: George Krajcsovits <krajorama@users.noreply.github.com>
Co-authored-by: Bartlomiej Plotka <bwplotka@gmail.com>

Author: krajorama