Vague spec: Span attributes may be "numeric"

[Oberon00]

See https://github.com/open-telemetry/opentelemetry-specification/blame/2316771e7e0ca3bfe9b2286d13e3a41ded6b8858/specification/api-tracing.md#L366:

(Required) The attribute value, which is either:

• A primitive type: string, boolean or numeric.
  ...

"Numeric" is very vague. Do we allow integers? How big? Floats? Which precision? Rationals? Decimals? Imaginary numbers? (Python even has literals for them).

I think it would be best to recommend what (not) to support. For example, the current opentelemetry-proto supports signed int64 and IEEE double.

[carlosalberto]

The integer/float case is relatively simple, as it has been implicitly taken for granted, and we should cook a PR making this part clear.

Not sure on the size/precision side though. Should we define that based on the OTel protos? Guess not quite?

[Oberon00]

We could take a C++-standard-like approach here and guarantee only a minimum set that protocols should support, like "signed and at least 32 bit int" , "IEEE float or double".

[carlosalberto]

guarantee only a minimum set that protocols should support

This should be good enough, I feel.

[jmacd]

The state of the spec is here: https://github.com/open-telemetry/opentelemetry-proto/blob/master/opentelemetry/proto/common/v1/common.proto

It uses IEEE double and int64. I would be happy if complex numbers and larger numbers were encoded as strings.