How could i return dynamic attributes in a sampler

Before opening a feature request against this repo, consider whether the feature should/could be implemented in the other OpenTelemetry client libraries. If so, please open an issue on opentelemetry-specification first.

Is your feature request related to a problem?

::opentelemetry::sdk::trace::SamplingResult MySampler::ShouldSample(
    const ::opentelemetry::trace::SpanContext& parent_context,
    ::opentelemetry::trace::TraceId trace_id, ::opentelemetry::nostd::string_view name,
    ::opentelemetry::trace::SpanKind span_kind,
    const ::opentelemetry::common::KeyValueIterable& attributes,
    const ::opentelemetry::trace::SpanContextKeyValueIterable& links) noexcept 

    result.attributes =
        std::make_unique<const std::map<std::string, opentelemetry::common::AttributeValue>>(
            std::move(kv));

SampleingResult has attributes, but it's value is AttributeValue, just reference, not own memory.

so, my question is, where could i attach original value string to this session, so it could free'd after span send.

parent_context?
trace_state?

no way !!! this API just could not make it work but still keeps it exist.

i think about thread_local, object pool, delay timer, std::string allocator, but all of this just make a simple things becomes very complex。

Describe the solution you'd like
no idea, dead lock.

Describe alternatives you've considered

Additional context