The instances of SpanContext should be immutable

[codeboten]

To be compliant with the tracing spec, the instances of SpanContext should be immutable

https://github.com/open-telemetry/opentelemetry-specification/blob/master/specification/trace/api.md#spancontext

[JasonXZLiu]

Hi! Is this issue free to pick up? I would love to work on it! -cc @alolita