Clarify scalar value types in Logs Data Model

[pellared]

The Logs Data Model is sparing describing the scalar values and it misses more information about the types.

See: https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/logs/data-model.md#type-any

I think that the specification should be precise similarly to: https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/common/README.md#attribute.

Some logging libraries make it possible to pass a uint64 attribute. However, it looks that it is not really possible to correctly pass such value via OTLP. But other exporters may able to handle such value so maybe uint64 should be supported as well?

CC @open-telemetry/specs-logs-approvers

Related PR: pellared/opentelemetry-go#7

[pellared]

CC @lalitb