Metrics SDK collector support: support "transient descriptors"

[jmacd]

There is a change I would like to submit for the Metrics SDK that does not directly address any SDK or API-level specification. What it will do is enable us to use the OTel-Go metrics SDK as part of an OTel-Collector processing pipeline.

The problem being solved is that the SDK expects each instrument used by the API to be registered once, statically. It expects that each instrument will have a unique registration, too. To use the SDK in a different context, where it is possible that multiple descriptors for the same name exist and where instruments are not long lived, requires support for "transient descriptors".

I have prototyped this code here:

jmacd#59

This re-uses the "refcount_mapped.go" support use to manage synchronous records for managing instruments. This code allows the OTel-Go SDK to be used in an environment where multiple descriptors with the same-name exist, and it will avoid leaking memory over time as these descriptors come and go. This also allows the OTel-Go SDK to be re-used as the implementation for other metric APIs with different instrument-lifecycle requirements than OTel, e.g., statsd libraries that do not require instrument registration.

[jmacd]

Connected with

open-telemetry/opentelemetry-specification#1046

[MrAlias]

@jmacd should we tag this with required-for-ga?