Support identical instrument registration and handle conflicts

[alanwest]

We currently reject instruments with duplicate names - that is, we log out a warning.

opentelemetry-dotnet/src/OpenTelemetry/Metrics/MetricReaderExt.cs

Lines 44 to 48 in 102ac27

	if (this.metricStreamNames.Contains(metricStreamName))
	{
	OpenTelemetrySdkEventSource.Log.MetricInstrumentIgnored(metricName, instrument.Meter.Name, "Metric name conflicting with existing name.", "Either change the name of the instrument or change name using View.");
	return null;
	}

Identical instrument registrations (name, kind, unit, description, and data type - int, double, etc) need to be allowed.

When instruments are not identical but are duplicated by name a warning should still be produced. Though, a new metric stream should be created.

Related spec change

Relevant sections of the spec:

• https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/metrics/api.md#instrument-type-conflict-detection
• https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/metrics/datamodel.md#opentelemetry-protocol-data-model-producer-recommendations
• https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/metrics/sdk.md#resolving-duplicate-instrument-registration-conflicts