Feature: configurable span tracking

[garthk]

Second attempt at implementing a configurable behaviour for span context tracking (open-telemetry/opentelemetry-erlang#4). This API shape felt more comfortable, especially how little change it took to switch with_child_span/3 across:

previous_span_ctx = Opencensus.Trace.get_span_ctx()
parent_span_ctx = Opencensus.Trace.effective_span_ctx()
new_span_ctx = :oc_trace.start_span("...", parent_span_ctx, %{...: :...})
Opencensus.Trace.put_span_ctx(new_span_ctx)
:...
:oc_trace.finish_span(new_span_ctx)
Opencensus.Trace.put_span_ctx(previous_span_ctx)

[codecov-io]

Codecov Report

Merging #26 into fix/span-capture-spillage-#22 will increase coverage by 0.3%.
The diff coverage is 100%.

@@                       Coverage Diff                        @@
##           fix/span-capture-spillage-#22      #26     +/-   ##
================================================================
+ Coverage                          98.24%   98.55%   +0.3%     
================================================================
  Files                                  5        6      +1     
  Lines                                 57       69     +12     
================================================================
+ Hits                                  56       68     +12     
  Misses                                 1        1

Impacted Files	Coverage Δ
lib/opencensus/process_context.ex	100% <100%> (ø)
lib/opencensus/trace.ex	96.87% <100%> (+0.87%)	⬆️

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