metrics: support labeled gauges

[algorandskiy]

Summary

Generalize counter code that supports labeled and non-labeled values into a new common couge (COUnter + gaUGE) type. This is needed by p2p branch to export number of labeled metrics.

Existing behavior change:

1. values without tags now exported as metric_name 123 instead of metric_name{} 123

Test Plan

Added new unites tests.

[codecov]

Codecov Report

Attention: Patch coverage is 97.60000% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 55.74%. Comparing base (dd22cff) to head (a1ad89f).
Report is 17 commits behind head on master.

Files	Patch %	Lines
util/metrics/couge.go	97.97%	1 Missing and 1 partial ⚠️
util/metrics/gauge.go	93.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5956      +/-   ##
==========================================
+ Coverage   55.71%   55.74%   +0.03%     
==========================================
  Files         489      490       +1     
  Lines       68111    68124      +13     
==========================================
+ Hits        37946    37979      +33     
+ Misses      27590    27577      -13     
+ Partials     2575     2568       -7     

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