Metrics, Prometheus example, Upgrade to Go 1.15 and fixes

[alitto]

• Expose comprehensive metrics through worker pool instances
• Add getters for all pool configuration options (max capacity, min and max workers and strategy)
• Upgrade golang runtime to 1.15
• Fix issue in TrySubmit() that caused tasks not to be queued when using a buffered pool and its max number of workers had been reached.

[codecov-io]

Codecov Report

Merging #9 (5b37421) into master (5f7cd69) will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master        #9   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            2         2           
  Lines          241       162   -79     
=========================================
- Hits           241       162   -79     

Impacted Files	Coverage Δ
pond.go	100.00% <100.00%> (ø)
resizer.go	100.00% <0.00%> (ø)

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