Does the sampler instance always need to return the same attributes per decision?

[Oberon00]

Related to #634.

In https://github.com/open-telemetry/opentelemetry-specification/blob/bc272ab52869df2145065d55300d721ff008e34b/specification/trace/sdk.md#shouldsample, the spec says:

The list of attributes returned by SamplingResult MUST be immutable. Caller may call this method any number of times and can safely cache the returned value.

I'm not sure what is meant by this. Does the same sampler instance always need to return the same attributes for the same decision? That would seem very restrictive. Or is this just specifying that the runtime instance returned needs to be immutable? If the latter, I think this is an implementation detail and should not be added to the specification, because it is language dependent. We also don't say for the attributes "string values must be copied so that modifications to them are not reflected on the Span", which would make sense e.g. for C++ with its modifiable std::strings.

CC @alolita, should I assign this to you too?

[bogdandrutu]

@Oberon00 it means the object returned is immutable, not that every time returns the same object.

[bogdandrutu]

Happy to remove that, maybe over-specify :)

[Oberon00]

I would be OK with removing it, but making more clear what is meant would also be fine.