feat(om2): relax the rules around unit and total suffixes

[krajorama]

Fixes: prometheus/OpenMetrics#286
Fixes: prometheus/OpenMetrics#282

[krajorama]

I agree with this comment from @beorn7 that the metric family name should be the same in the metadata and what we write in the metric(point)s.

# TYPE foo_total counter
# HELP foo_total A help string.
foo_total 42

or

# TYPE foo counter
# HELP foo A help string.
foo 42

but to be able to make a generic statement, we need these to be fixed:
prometheus/OpenMetrics#285 Created timestamp not exposed as "foo_create" anymore.
prometheus/OpenMetrics#283 Complex types for classic histograms and summaries.

[dashpole]

FWIW, prometheus/OpenMetrics#286 and prometheus/OpenMetrics#282 only ask for it to be allowed to not use suffixes. IMO the primary use-case for allowing the same name, but different unit and type is for federation-like endpoints (e.g. the prometheusexporter in the collector) where metric data from multiple applications is combined. We could require OTel SDK users to resolve naming collisions using views. I'm still OK with this change, but want to make sure we are clear on what the requirements from OTel are.

[dashpole]

Suggested change

	MetricFamily names SHOULD be uniq within a MetricSet. This rule relaxes the requirement in previous versions of OpenMetrics and is intended to facilitate exposing OpenTelemetry data model. For other use cases it is highly recommended to continue to adhere to this rule.
	MetricFamily names SHOULD be unique within a MetricSet. This rule relaxes the requirement in previous versions of OpenMetrics and is intended to facilitate exposing OpenTelemetry data model. For other use cases it is highly recommended to continue to adhere to this rule.