[spec compliance] Metrics SDK does not fail on duplicate name/different instrument metrics.

[jsuereth]

Currently, the metric storage component does not fail if in the current scenario:

• There are two different instruments (e.g. Counter + Histogram) with the same name on the same meter
• A user provided View configuration renames the Counter to another name with no conflict.

This should fail, according to the specification.

[jack-berg]

There are two different instruments (e.g. Counter + Histogram) with the same name on the same meter

@jsuereth I believe this is no longer possible now that MetricDescriptor#isCompatible(MetricDescriptor) checks that the instruments are of the same type. The user should get a compatibility warning log if they try to do this.

[jack-berg]

Closing because the spec has clarified the behavior in the presence of identity conflicts. This was implemented in #4222.