loadbalancingexporter: add Start for exporters

[pkositsyn]

Description:
Fixed a bug with not working loadbalancingexporter with retry queue. Also added sorting for endpoints on backend change (assume it is occasionally missing).

Link to tracking Issue: Fixes #1621

Testing:
Added test for endpoints sort

[pkositsyn]

I don't know whether its ok to save host for later usage. @bogdandrutu Please verify that it's legal

[pkositsyn]

Also I think this kind of bug is really painful. Can we prevent such things in future? Introduce unified end-to-end test for every new receiver and exporter or something.

[codecov]

Codecov Report

Merging #1687 (3492288) into master (170cac0) will decrease coverage by 0.01%.
The diff coverage is 60.00%.

@@            Coverage Diff             @@
##           master    #1687      +/-   ##
==========================================
- Coverage   89.39%   89.37%   -0.02%     
==========================================
  Files         370      370              
  Lines       18115    18118       +3     
==========================================
  Hits        16193    16193              
- Misses       1432     1434       +2     
- Partials      490      491       +1     

Flag	Coverage Δ
integration	70.97% <ø> (ø)
unit	88.06% <60.00%> (-0.02%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
exporter/loadbalancingexporter/exporter.go	92.23% <60.00%> (-2.77%)	⬇️

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[bogdandrutu]

@Vemmy124 we don't have any concern for the moment about not keeping a reference to the host. So it is ok.

[pkositsyn]

@jpkrohling Resolved the issue in the latest commit