Send telemetry for environment variables used per extension call

[larohra]

Description

Send telemetry for environment variables used per extension call

Issue #

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PR information

• The title of the PR is clear and informative.
• There are a small number of commits, each of which has an informative message. This means that previously merged commits do not appear in the history of the PR. For information on cleaning up the commits in your pull request, see this page.
• If applicable, the PR references the bug/issue that it fixes in the description.
• New Unit tests were added for the changes made

Quality of Code and Contribution Guidelines

• I have read the contribution guidelines.

[codecov]

Codecov Report

Merging #2536 (64c4bf1) into develop (be720fe) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop    #2536      +/-   ##
===========================================
- Coverage    71.99%   71.98%   -0.01%     
===========================================
  Files          102      102              
  Lines        15192    15194       +2     
  Branches      2403     2403              
===========================================
+ Hits         10937    10938       +1     
- Misses        3773     3775       +2     
+ Partials       482      481       -1     

Impacted Files	Coverage Δ
azurelinuxagent/ga/exthandlers.py	85.71% <100.00%> (+0.02%)	⬆️
azurelinuxagent/common/event.py	85.36% <0.00%> (-0.68%)	⬇️
azurelinuxagent/ga/collect_telemetry_events.py	90.41% <0.00%> (+0.68%)	⬆️

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