Attribute name for defining observability signal (data type).

[dmitryax]

Recently, we added an attribute to represent an OTel data type (metrics/traces/logs) to an internal collector metric in open-telemetry/opentelemetry-collector#10593. The semantic conventions do not provide guidance on what the attribute should be called. The WIP OTEP suggests calling it otel.signal. Defining a name for this attribute is useful independently of the OTEP.

otel.signal should like a good option. Looking for other opinions.