fix(langchain): Make span_map an instance variable

[szokeasaurusrex]

Ref #4443

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.54%. Comparing base (0e21fa2) to head (906f75f).

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4476      +/-   ##
==========================================
- Coverage   80.59%   80.54%   -0.06%     
==========================================
  Files         142      142              
  Lines       16030    16029       -1     
  Branches     2745     2745              
==========================================
- Hits        12920    12910      -10     
- Misses       2243     2250       +7     
- Partials      867      869       +2     

Files with missing lines	Coverage Δ
sentry_sdk/integrations/langchain.py	66.66% <100.00%> (-1.37%)	⬇️

... and 2 files with indirect coverage changes