More implementation for io.opentelemetry.api.common.AttributeKey

Currently, we have only one implementation io.opentelemetry.api.internal.InternalAttributeKeyImpl for io.opentelemetry.api.common.AttributeKey, and we may have more implementation for that, for example:

1. Make the all attributes in io.opentelemetry.semconv.trace.attributes.SemanticAttributes SemanticAttributeImpl

public class SemanticAttributeImpl<T> implements AttributeKey<T> {}

2. Make the SemanticAttributes that can be recored into metrics MetricsSemanticAttributes

public class MetricsSemanticAttributes<T> extends SemanticAttributeImpl {}

3. Make the other attributes CustomAttributeImpl

If AttributeKey is classified, we can do a lot of performance optimisation. For example, we can filter out MetricsAttributes of SemanticAttributes by just calling xx.getclass().equals(MetricsAttributes) instead of traversing it.

Start by reading io.opentelemetry.api.common.AttributeKey, io.opentelemetry.api.internal.InternalAttributeKeyImpl, and io.opentelemetry.semconv.trace.attributes.SemanticAttributes. Clarify the proposed classification of semantic, metrics, and custom attributes, then determine the expected performance behavior and validation needed for the new implementations.