fix(om2): consistency between model and exposition

The model said that: A MetricFamily MUST have a name, HELP, TYPE, and UNIT
 metadata. However the exposition does not require it to be exposed,
therefore for the sake of consistency we make this SHOULD.
Also having all caps conflated the model with the exposition.

Also this allows very simple exposers to not think about this.

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

Author: krajorama

docs/specs/om/open_metrics_spec_2_0.md

@@ -148,7 +148,7 @@ If more than one MetricPoint is exposed for a Metric, then its MetricPoints MUST
 
 #### MetricFamily
 
-A MetricFamily MAY have zero or more Metrics. A MetricFamily MUST have a name, HELP, TYPE, and UNIT metadata. Every Metric within a MetricFamily MUST have a unique LabelSet.
+A MetricFamily MAY have zero or more Metrics. Every Metric within a MetricFamily MUST have a unique LabelSet. A MetricFamily SHOULD have a name, Help, Type, and Unit metadata.
 
 ##### Name