Solve the problem that Span cannot be closed normally when Http.Client is a singleton

[Matrix-K]

As the title says

[codecov-io]

Codecov Report

Merging #74 into master will decrease coverage by 2.65%. The diff coverage is 61.53%.

@@            Coverage Diff             @@
##           master      #74      +/-   ##
==========================================
- Coverage   69.79%   67.14%   -2.66%     
==========================================
  Files          14       13       -1     
  Lines         682      560     -122     
==========================================
- Hits          476      376     -100     
+ Misses        165      150      -15     
+ Partials       41       34       -7     

Impacted Files	Coverage Δ
span.go	100.00% <ø> (ø)
trace_opts.go	30.76% <0.00%> (-69.24%)	:arrow_down:
trace.go	78.35% <61.53%> (-4.99%)	:arrow_down:
plugins/http/client.go	47.61% <66.66%> (+1.78%)	:arrow_up:
plugins/http/server.go	50.00% <100.00%> (+3.84%)	:arrow_up:
sampler.go	100.00% <100.00%> (ø)
span_opts.go	100.00% <100.00%> (ø)
noop.go	66.66% <0.00%> (-19.05%)	:arrow_down:
propagation/propagation.go	62.50% <0.00%> (-3.54%)	:arrow_down:
reporter/grpc.go	59.41% <0.00%> (-1.93%)	:arrow_down:
... and 10 more

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[arugal]

@Matrix-K any update?

[wu-sheng]

@Matrix-K We need your update, otherwise, we have to close this PR.