Add server unsubscribes metric

[FZambia]

New metric:

prometheus.NewCounterVec(prometheus.CounterOpts{
	Namespace: metricsNamespace,
	Subsystem: "client",
	Name:      "num_server_unsubscribes",
	Help:      "Number of server initiated unsubscribes.",
}, []string{"code"})

Collected for each server-side unsubscribe push going to the client.

[codecov]

Codecov Report

Attention: Patch coverage is 69.23077% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 81.79%. Comparing base (5dd6664) to head (1ee1474).

Files	Patch %	Lines
client.go	33.33%	2 Missing ⚠️
metrics.go	80.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #383      +/-   ##
==========================================
+ Coverage   81.75%   81.79%   +0.03%     
==========================================
  Files          40       40              
  Lines        7033     7046      +13     
==========================================
+ Hits         5750     5763      +13     
  Misses        920      920              
  Partials      363      363              

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