Initial relocation of ContextUtils classes.

[carlosalberto]

Now they will live within their own child propagation package (instead of unsafe), with a prefix based on their cross-cutting concern, to be referenced easily.

NOTE: Context.Key instances now don't have a default value, and we provide an actual default when they return null. The reason for this is that, upon Injection, we want to detect whether there's an actual value in Context:

// Inject
Span span = CONTEXT_SPAN_KEY.get(Context.current());
if (span == null) {
  // Nothing to inject.
  return;
}

An alternative would be to check against DefaultSpan.getInvalid() if we offer such default value along the key.

Because of this, we offer getSpan() and getSpanWithDefault() variants (SpanBuilderSdk uses the former to easily check that Span is not a no-op instance, for example).

I'm open to discuss this detail though.

This has been split from #720

[carlosalberto]

Updated ;)

@bogdandrutu Left a comment, please review.

[codecov-io]

Codecov Report

Merging #904 into master will decrease coverage by 0.1%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master     #904      +/-   ##
============================================
- Coverage     84.39%   84.28%   -0.11%     
+ Complexity      893      890       -3     
============================================
  Files           119      118       -1     
  Lines          3165     3163       -2     
  Branches        271      273       +2     
============================================
- Hits           2671     2666       -5     
- Misses          379      380       +1     
- Partials        115      117       +2

Impacted Files	Coverage Δ	Complexity Δ
...va/io/opentelemetry/trace/TracingContextUtils.java	100% <100%> (ø)	7 <7> (?)
...ava/io/opentelemetry/sdk/trace/SpanBuilderSdk.java	93.93% <100%> (ø)	36 <4> (ø)	⬇️
...ationcontext/DefaultCorrelationContextManager.java	69.23% <100%> (ø)	7 <2> (ø)	⬇️
.../opentelemetry/contrib/trace/CurrentSpanUtils.java	85.71% <100%> (ø)	4 <0> (ø)	⬇️
...y/correlationcontext/CorrelationsContextUtils.java	100% <100%> (ø)	7 <7> (?)
...in/java/io/opentelemetry/context/ContextUtils.java	100% <100%> (ø)	2 <2> (?)
...ain/java/io/opentelemetry/sdk/trace/TracerSdk.java	100% <100%> (ø)	8 <2> (ø)	⬇️
...rrelationcontext/CorrelationContextManagerSdk.java	100% <100%> (ø)	5 <2> (ø)	⬇️
...ain/java/io/opentelemetry/trace/DefaultTracer.java	95.23% <100%> (ø)	6 <2> (ø)	⬇️
...ntelemetry/sdk/trace/RecordEventsReadableSpan.java	90.3% <0%> (-1.22%)	44% <0%> (-1%)
... and 4 more

---

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[carlosalberto]

@open-telemetry/java-approvers This PR has been updated:

• Have the default versions return a no-op instance if none was found, and having an extra get[value]WithoutDefault() to return no default values (used mostly by propagators).
• Removed for now the SpanContext bits to also store/retrieve it from Context.

[carlosalberto]

Thanks for the feedback @bogdandrutu Please review the latest questions I left, and I will iterate ASAP ;)

[bogdandrutu]

Only one comment left to remove the version of the withSpan without the context (same for correlation).