feat(metric): add success, failure and submitted tasks

[appleboy]

type metric struct {
	busyWorkers    uint64
	successTasks   uint64
	failureTasks   uint64
	submittedTasks uint64
}

fix #47

Signed-off-by: Bo-Yi Wu appleboy.tw@gmail.com

[codecov-commenter]

Codecov Report

Merging #48 (167ef13) into master (8af744c) will decrease coverage by 1.04%.
The diff coverage is 94.73%.

@@            Coverage Diff             @@
##           master      #48      +/-   ##
==========================================
- Coverage   85.81%   84.77%   -1.05%     
==========================================
  Files          10       10              
  Lines         416      440      +24     
==========================================
+ Hits          357      373      +16     
- Misses         54       62       +8     
  Partials        5        5              

Flag	Coverage Δ
go-1.13	84.77% <94.73%> (-0.81%)	⬇️
go-1.14	84.77% <94.73%> (-0.81%)	⬇️
go-1.15	84.77% <94.73%> (-1.05%)	⬇️
go-1.16	84.77% <94.73%> (-0.81%)	⬇️
go-1.17	84.77% <94.73%> (-0.81%)	⬇️
go-1.18	84.77% <94.73%> (-0.81%)	⬇️
ubuntu-latest	84.77% <94.73%> (-1.05%)	⬇️

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Impacted Files	Coverage Δ
worker_empty.go	25.00% <ø> (-3.58%)	⬇️
worker_message.go	45.00% <ø> (-41.96%)	⬇️
worker_task.go	90.47% <ø> (+7.14%)	⬆️
queue.go	86.12% <93.33%> (-0.72%)	⬇️
metric.go	100.00% <100.00%> (ø)

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