ProxyMeter must return proxy instruments or users may get stale instruments in the API

[aabmass]

Currently from #1887, the ProxyMeter will defer to a global MeterProvider once it is set in the API. However, this needs to go all the way down to the instrument level, or users will end up with no-op instruments even after configuring an SDK.

[ocelotl]

I think this would be consistent with the general proxy approach, but it would be inconsistent with the current approach that we have implemented in Tracing, for example:

TracerProvider produces Tracers -> MeterProvider produces Meters
Tracer produce Spans -> Meter produce Instruments

We don't have ProxySpans, we have invalid spans.

I see how this approach is problematic as well, users may end up with no-op spans even after configuring an SDK.

I have the feeling that we should allow Tracer/MeterProviders to be changed (again) until we have a more well defined plan, because this seems like will conflict with the upcoming work that should introduce configuration and/or distros.

[aabmass]

I see how this approach is problematic as well, users may end up with no-op spans even after configuring an SDK.

It isn't quite the same, as spans are generally short lived while instruments are part of the initial application set-up. With this approach, the user will still lose measurements made before the SDK is configured, just how they would lose spans. I think it's OK to lose a little bit of telemetry in this edge case where a user creates tracers/instruments before setting up the SDK.