Polish api.Tracer with new error tracing mechanism

[sczyh30]

Signed-off-by: Eric Zhao sczyh16@gmail.com

Describe what this PR does / why we need it

Polish api.Tracer with new error tracing mechanism (set error to entry and record on request completed).

Does this pull request fix one issue?

Related to #143

[codecov-commenter]

Codecov Report

Merging #153 into master will decrease coverage by 0.06%.
The diff coverage is 54.54%.

@@            Coverage Diff             @@
##           master     #153      +/-   ##
==========================================
- Coverage   41.64%   41.57%   -0.07%     
==========================================
  Files          68       68              
  Lines        3314     3495     +181     
==========================================
+ Hits         1380     1453      +73     
- Misses       1799     1906     +107     
- Partials      135      136       +1     

Impacted Files	Coverage Δ
adapter/dubbo/consumer_filter.go	48.38% <0.00%> (ø)
adapter/dubbo/provider_filter.go	48.38% <0.00%> (ø)
api/tracer.go	37.50% <66.66%> (+1.78%)	⬆️
adapter/grpc/client.go	92.59% <100.00%> (ø)
adapter/grpc/server.go	84.00% <100.00%> (ø)
core/flow/tc_throttling.go	66.66% <0.00%> (-11.12%)	⬇️
core/circuitbreaker/rule.go	88.73% <0.00%> (-3.65%)	⬇️
core/circuitbreaker/stat_slot.go	0.00% <0.00%> (ø)
core/circuitbreaker/circuit_breaker.go	23.87% <0.00%> (+0.56%)	⬆️
core/circuitbreaker/rule_manager.go	53.24% <0.00%> (+1.16%)	⬆️

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