feat(pymongo): Send query description as valid JSON

[0Calories]

MongoDB queries were being sent as invalid JSON, since the keys and values were surrounded by single quotes instead of double quotes. Relay cannot parse the queries unless they are sent as valid JSON. This PR converts MongoDB queries into a JSON string before sending it on the span, so that Relay may properly parse it and extract metrics.

Before

After

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.41%. Comparing base (301c4b8) to head (8adea02).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3291   +/-   ##
=======================================
  Coverage   79.40%   79.41%           
=======================================
  Files         132      132           
  Lines       14281    14282    +1     
  Branches     2999     2999           
=======================================
+ Hits        11340    11342    +2     
  Misses       2094     2094           
+ Partials      847      846    -1     

Files	Coverage Δ
sentry_sdk/integrations/pymongo.py	84.16% <100.00%> (+0.13%)	⬆️

... and 1 file with indirect coverage changes