Migrate metrics from Opencensus to Opentelemetry

[s-radyuk]

This PR migrates Kangal from using Opencensus to Opentelemetry:

• both proxy and controller use go.opentelemetry Prometheus exporter
• countRunningLoadtests metric collected asynchronously in proxy
• reconciler tag is removed from metrics, as we have only one reconciler in code and it's always the same
• all server methods use otelhttp.NewHandler to collect http metrics
• no new logic introduced

[codecov-commenter]

Codecov Report

Merging #245 (8e2155c) into master (bcedc5a) will decrease coverage by 0.98%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #245      +/-   ##
==========================================
- Coverage   56.87%   55.88%   -0.99%     
==========================================
  Files          37       37              
  Lines        4032     4103      +71     
==========================================
  Hits         2293     2293              
- Misses       1615     1686      +71     
  Partials      124      124              

Impacted Files	Coverage Δ
cmd/controller.go	20.37% <0.00%> (-2.08%)	⬇️
cmd/proxy.go	0.00% <0.00%> (ø)
pkg/controller/controller.go	0.00% <0.00%> (ø)
pkg/controller/loadtest.go	2.99% <0.00%> (-0.18%)	⬇️
pkg/controller/metrics.go	0.00% <0.00%> (ø)
pkg/kubernetes/client.go	83.75% <0.00%> (-6.80%)	⬇️
pkg/proxy/proxy.go	78.72% <0.00%> (-6.93%)	⬇️
pkg/proxy/server.go	0.00% <0.00%> (ø)

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