Instrument name collisions for different Meters

[pirgeo]

Opening this as a tracking issue for the bug where Instruments that have the same name are dropped, even if they come from separate Meters. From the API spec:

Different Meters MUST be treated as separate namespaces. The names of the Instruments under one Meter SHOULD NOT interfere with Instruments under another Meter.

This issue does only cover the fact that duplicate instrument elimination in the .NET SDK implementation does not take into account the Meter name, but only the Instrument name. It does not cover downstream implications (such as how to deal with Prometheus export in such a case, which has extensively been discussed and resolved (?) elsewhere).

While this should occur only rarely, I think its important that this is addressed. As more and more libraries adopt OpenTelemetry, such situations are likely to appear more often. OTLP is able to transport these data streams, so it seems counter-intuitive to drop them.

Additional Context:

https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/metrics/api.md#instrument

open-telemetry/opentelemetry-specification#2064
Code where instruments are dropped when using Views
Code where instruments are dropped without Views

[pirgeo]

Potentially relevant for #2621

[cijothomas]

Thanks! The fix would be small one, so adding to 1.2 milestone for now.
@joaopgrassi who initially raised the concern.