Prometheus metrics scraper

[vbehar]

This change decouples the metrics collection and scraping, and allows custom scrapers.
The default scraper stays the osiris one, using the auto-injected sidecar container.

And we introduce a new scraper, using the prometheus format. It doesn't require the osiris proxy,
and instead scrap metrics from a application-provided prometheus-compliant endpoint.

see #48 for more details

[codecov-io]

Codecov Report

Merging #54 into master will decrease coverage by 21.75%.
The diff coverage is 21.11%.

@@             Coverage Diff             @@
##           master      #54       +/-   ##
===========================================
- Coverage   59.09%   37.33%   -21.76%     
===========================================
  Files          11       17        +6     
  Lines         638     1192      +554     
===========================================
+ Hits          377      445       +68     
- Misses        232      716      +484     
- Partials       29       31        +2

Impacted Files	Coverage Δ
pkg/kubernetes/osiris.go	100% <ø> (ø)	⬆️
pkg/deployments/zeroscaler/metrics_collector.go	0% <0%> (ø)
pkg/deployments/zeroscaler/metrics_scraper.go	0% <0%> (ø)
pkg/deployments/zeroscaler/osiris_scraper.go	23.07% <23.07%> (ø)
pkg/deployments/zeroscaler/prometheus_scraper.go	24.55% <24.55%> (ø)
pkg/deployments/zeroscaler/zeroscaler.go	8.77% <25%> (ø)
pkg/deployments/zeroscaler/config.go	0% <0%> (ø)
... and 2 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update a197da3...de8c9e7. Read the comment docs.