Create a standard way to create a map of metric identity

[alanwest]

This was proposed by @jmacd in the context of the cumulative to delta processor from the contrib repo. open-telemetry/opentelemetry-collector-contrib#4444 (review)

There ought to be a standard way to create a map of metric identity, because more than one processor is going to use such a thing. The OTel-Go SDK has such a thing, but it's not perfectly simple to adapt. The tracking entry here could be replaced by a struct that is usable as a map key w/o the Write() mechanism, but I wouldn't hold up this PR because of it. The collector should support something like the OTel-Go attribute.Distinct type that applies to model/pdata types so that code like this can easily create a map key based on resource, instrumentation library, name, unit and attributes.