Replace trac(k)ing abstract with more generic context tracking

[bsideup]

After #1492, there is no need for a special Marker abstraction and the Tracing story can be simplified to "Context tracking".

[simonbasle]

needs small doc improvements, but great to be able to remove unnecessary interface ;)

[simonbasle]

snippet still references Marker

[bsideup]

That's an example, Marker is something provided by the implementation

[simonbasle]

maybe flesh out this sentence: the method would extract key-value pairs from Context and put them in appropriate ThreadLocal variables, then return a Disposable that clears said threadlocals. Maybe ThreadLocal is not the right example either (MDC ?).

[bsideup]

Most tracers use ThreadLocal to store the spans (as well as MDC's implementation), hence the mention :)

But I like the idea of guiding how to implement it, will add 👍

[codecov-io]

Codecov Report

Merging #1495 into tracing will decrease coverage by 0.06%.
The diff coverage is 74.28%.

@@              Coverage Diff              @@
##             tracing    #1495      +/-   ##
=============================================
- Coverage       83.9%   83.84%   -0.07%     
+ Complexity      3932     3918      -14     
=============================================
  Files            363      363              
  Lines          30198    29955     -243     
  Branches        5601     5527      -74     
=============================================
- Hits           25339    25117     -222     
+ Misses          3208     3192      -16     
+ Partials        1651     1646       -5

Impacted Files	Coverage Δ	Complexity Δ
...in/java/reactor/core/publisher/ContextTracker.java	0% <0%> (ø)	0 <0> (?)
...isher/ContextTrackingExecutorServiceDecorator.java	45.94% <55.55%> (ø)	2 <1> (?)
...actor/core/publisher/ContextTrackingPublisher.java	73.68% <83.33%> (ø)	4 <4> (?)
...re/src/main/java/reactor/core/publisher/Hooks.java	92% <84.61%> (ø)	49 <2> (ø)	⬇️
...e/src/main/java/reactor/core/publisher/Traces.java	66.66% <0%> (-6.16%)	25% <0%> (-8%)
...ain/java/reactor/core/publisher/MonoCacheTime.java	87.07% <0%> (-3.41%)	18% <0%> (-1%)
...a/reactor/core/scheduler/ReactorThreadFactory.java	85.71% <0%> (-1.25%)	6% <0%> (ø)
...ain/java/reactor/core/publisher/FluxUsingWhen.java	96.41% <0%> (-0.99%)	8% <0%> (-8%)
...c/main/java/reactor/core/publisher/RingBuffer.java	70.56% <0%> (-0.95%)	24% <0%> (ø)
...main/java/reactor/core/publisher/FluxGenerate.java	69.32% <0%> (-0.62%)	7% <0%> (ø)
... and 17 more

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