Update trace flags to match spec

[jtescher]

Remove all trace flags except sampled as that is the only supported flag in the current trace spec. Removes SpanContext::is_deferred and SpanContext::is_debug as they are also no longer supported.

Resolves #559

[TommyCpp]

Looks good! Just a small nit: I wonder if use !TraceFlags::SAMPLED in tests will be better than TraceFlags::default() as readers don't need to figure out what's the default of TraceFlags?

[codecov]

Codecov Report

Merging #565 (8fababf) into main (635f10e) will increase coverage by 0.0%.
The diff coverage is 94.2%.

❗ Current head 8fababf differs from pull request most recent head c47dcfb. Consider uploading reports for the commit c47dcfb to get more accurate results

@@          Coverage Diff          @@
##            main    #565   +/-   ##
=====================================
  Coverage   54.6%   54.7%           
=====================================
  Files         98      98           
  Lines       8550    8576   +26     
=====================================
+ Hits        4674    4694   +20     
- Misses      3876    3882    +6     

Impacted Files	Coverage Δ
opentelemetry-jaeger/src/exporter/mod.rs	47.8% <0.0%> (ø)
opentelemetry/src/global/trace.rs	17.0% <ø> (ø)
opentelemetry/src/trace/mod.rs	50.0% <ø> (ø)
opentelemetry/src/util.rs	27.7% <ø> (ø)
opentelemetry/src/sdk/trace/tracer.rs	85.9% <80.0%> (+0.1%)	⬆️
opentelemetry/src/trace/span_context.rs	91.7% <81.2%> (-1.8%)	⬇️
opentelemetry-aws/src/lib.rs	93.3% <100.0%> (ø)
opentelemetry-datadog/src/exporter/model/mod.rs	64.1% <100.0%> (+0.5%)	⬆️
opentelemetry-datadog/src/lib.rs	92.4% <100.0%> (ø)
opentelemetry-jaeger/src/lib.rs	94.1% <100.0%> (+0.1%)	⬆️
... and 9 more

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[jtescher]

@TommyCpp yeah little confusing. As TraceFlags is a bitset !TraceFlags::SAMPLED is the same as !0x01 which is 0xfe rather than 0x00. So what you would want would maybe be more like TraceFlags::default() & !TraceFlags::SAMPLED, but that's not super readable either. Looked at the other impls and it doesn't seem like others have an unsampled constant in the public API.

[joshtriplett]

This seems like an improvement. Would you consider adding documentation on SpanContext and on TraceFlags::SAMPLED that mentions that some tracing tools will completely ignore spans that don't have the sampled flag set?

[joshtriplett]

Perfect, thanks!

[jtescher]

@TommyCpp I can think more about test clarity and do a follow up PR if a better solution emerges there.