Check metrics server availability during canary initialization

[mathetake]

resolves #554

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TODO in this PR:

• write uni test for checkMetricProviderAvailability

[codecov-io]

Codecov Report

Merging #592 into master will decrease coverage by 0.02%.
The diff coverage is 48.71%.

@@            Coverage Diff             @@
##           master     #592      +/-   ##
==========================================
- Coverage   54.95%   54.92%   -0.03%     
==========================================
  Files          62       62              
  Lines        5241     5278      +37     
==========================================
+ Hits         2880     2899      +19     
- Misses       1944     1956      +12     
- Partials      417      423       +6     

Impacted Files	Coverage Δ
pkg/metrics/providers/prometheus.go	69.69% <ø> (+2.04%)	⬆️
pkg/controller/scheduler.go	44.67% <33.33%> (-0.08%)	⬇️
pkg/controller/scheduler_metrics.go	34.18% <50.00%> (+3.55%)	⬆️

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[mathetake]

will work on unit test tomorrow

[stefanprodan]

LGTM 💯