[Telemetry] Add eventlog endpoint for collecting client-side events

[kiendang]

for client telemetry events. originally from #364, moved to a separate PR here after discussion at jupyter-server/team-compass#4.

need to be in sync with #364 and cannot be merged before #364 is merged and jupyter/telemetry#62 is resolved.

[codecov-commenter]

Codecov Report

Merging #501 (e15dd9b) into master (678878f) will decrease coverage by 0.08%. The diff coverage is 66.40%.

@@            Coverage Diff             @@
##           master     #501      +/-   ##
==========================================
- Coverage   77.73%   77.64%   -0.09%     
==========================================
  Files         106      108       +2     
  Lines        9288     9412     +124     
  Branches     1001     1018      +17     
==========================================
+ Hits         7220     7308      +88     
- Misses       1710     1745      +35     
- Partials      358      359       +1     

Impacted Files	Coverage Δ
jupyter_server/services/eventlog/handlers.py	32.00% <32.00%> (ø)
jupyter_server/utils.py	60.81% <57.77%> (-1.33%)	:arrow_down:
jupyter_server/serverapp.py	67.12% <60.00%> (-0.12%)	:arrow_down:
jupyter_server/base/handlers.py	64.74% <100.00%> (+1.04%)	:arrow_up:
jupyter_server/pytest_plugin.py	92.14% <100.00%> (+0.43%)	:arrow_up:
jupyter_server/services/contents/handlers.py	84.93% <100.00%> (+0.76%)	:arrow_up:
jupyter_server/tests/test_eventlog.py	100.00% <100.00%> (ø)
... and 1 more

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[Zsailer]

This work has been superceded by #862. Closing here.