Use contextvars to maintain a call stack during the usage calls

[achals]

Signed-off-by: Achal Shah achals@gmail.com

What this PR does / why we need it:
Our usage handling doesn't handle calls to usage annotated events within the implementation of get_online_features. So, e.g., calling get_feature_views within get_online_features would always get called, which would result in bad performance.

This fixes that by maintaining the call stack and making sure any call path that includes get_online_features is only sampled.

Which issue(s) this PR fixes:

Fixes #

Does this PR introduce a user-facing change?:

none

[codecov-commenter]

Codecov Report

Merging #1882 (6a4f876) into master (efeea6c) will increase coverage by 0.13%.
The diff coverage is 94.11%.

@@            Coverage Diff             @@
##           master    #1882      +/-   ##
==========================================
+ Coverage   83.69%   83.82%   +0.13%     
==========================================
  Files          96       96              
  Lines        7353     7363      +10     
==========================================
+ Hits         6154     6172      +18     
+ Misses       1199     1191       -8     

Flag	Coverage Δ
integrationtests	74.55% <94.11%> (+0.12%)	⬆️
unittests	62.40% <88.23%> (+0.10%)	⬆️

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

Impacted Files	Coverage Δ
sdk/python/feast/usage.py	66.66% <94.11%> (+6.21%)	⬆️
sdk/python/feast/infra/online_stores/datastore.py	93.44% <0.00%> (+2.45%)	⬆️

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[adchia]

/lgtm

[feast-ci-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: achals, adchia

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