extending lt counter metric

[s-radyuk]

This PR extends the logic of existing CountRunningLoadtests to count all the tests currently existing in the cluster:

• rename metric to CountExistingLoadtests
• metric returns count of loadtests by type and by phase
• the same method CountExistingLoadtests now can be used instead of CountActiveLoadTests with duplicated logic

[codecov-commenter]

Codecov Report

Merging #250 (aa1211a) into master (5c98c24) will increase coverage by 0.23%. The diff coverage is 64.86%.

:exclamation: Current head aa1211a differs from pull request most recent head 500e3bb. Consider uploading reports for the commit 500e3bb to get more accurate results

@@            Coverage Diff             @@
##           master     #250      +/-   ##
==========================================
+ Coverage   55.88%   56.12%   +0.23%     
==========================================
  Files          37       37              
  Lines        4103     4116      +13     
==========================================
+ Hits         2293     2310      +17     
+ Misses       1686     1682       -4     
  Partials      124      124              

Impacted Files	Coverage Δ
pkg/proxy/proxy.go	76.63% <26.66%> (-2.09%)	:arrow_down:
pkg/kubernetes/client.go	90.85% <90.90%> (+7.10%)	:arrow_up:

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