Enable mutating sampled flag

[slashmo]

As part of an effort to enabling mutation of the sampled flag, I refactored traceParent.traceFlags to use an OptionSet. Because the W3C Trace Context spec only supports TraceFlags.sampled at this point, parent.traceFlags cannot be mutated directly. Instead, TraceContext itself now has a get/set sampled property that internally retrieves from/writes to parent.traceFlags.

When parsing TraceParent from its String representation, any traceFlags value higher than 01 will be ignored and traceFlags default to 00 (not sampled), as required in the spec:

The behavior of other flags, such as (00000100) is not defined and is reserved for future use. Vendors MUST set those to zero.

[slashmo]

@ktoso I'd be very interested in your opinion of this implementation 🙂

[ktoso]

Skimming it now and I think it looks good! I will give it another look once I get my hands on a MacBook later :)

[slashmo]

@ktoso Thanks for the review 👍