Fix instrument identity evaluation in MetricStorageRegistry#register

[jack-berg]

As discussed in this comment, there's a bug in our evaluation of instrument identity in MetricStorageRegistry#register(..).

MetricStorageRegistry uses MetricDescriptor#equals(..) to determine if an existing metric storage should be used with a newly registered instrument. If yes, the new instrument's measurements will be merged with the existing. If no, a new storage is created and the instrument's measurements are associated with a new metric stream at collection.

I know of two things we're doing wrong:

• If instruments are the same except for the case of a name, there should be a single metric stream exported, with the name of the first registered.
• If instruments are the same except for advice, there should be a single metric stream exported, with the advice of the first registered.

The proof of the first issue is that the following test code fails:

    InMemoryMetricReader reader = InMemoryMetricReader.create();
    meterProvider = SdkMeterProvider.builder().registerMetricReader(reader).build();

    meterProvider.get("meter").counterBuilder("Counter").ofDoubles().build().add(1);
    meterProvider.get("meter").counterBuilder("counter").ofDoubles().build().add(1);

    assertThat(reader.collectAllMetrics())
        .satisfiesExactly(
            metric ->
                assertThat(metric)
                    .hasName("Counter")
                    .hasDoubleSumSatisfying(
                        sum ->
                            sum.hasPointsSatisfying(
                                point ->
                                    point.hasValue(2))));

Can produce similar test code to illustrate the second issue.