fix(axiom): externalCallCount should be added to the txn not just the error

[zsistla]

No description provided.

[newrelic-php-agent-bot]

Test Suite	Status	Result
Multiverse	✅	8/8 passing
SOAK	✅	72/72 passing

[lavarou]

LGTM

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.04%. Comparing base (f26aead) to head (28b7e59).
Report is 1 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #1006      +/-   ##
==========================================
- Coverage   78.61%   78.04%   -0.57%     
==========================================
  Files         196      196              
  Lines       27069    27105      +36     
==========================================
- Hits        21280    21155     -125     
- Misses       5789     5950     +161     

Flag	Coverage Δ
agent-for-php-7.2	78.04% <100.00%> (-0.59%)	⬇️
agent-for-php-7.3	78.05% <100.00%> (-0.59%)	⬇️
agent-for-php-7.4	77.76% <100.00%> (-0.59%)	⬇️
agent-for-php-8.0	77.13% <100.00%> (-1.24%)	⬇️
agent-for-php-8.1	77.79% <100.00%> (-0.56%)	⬇️
agent-for-php-8.2	77.38% <100.00%> (-0.56%)	⬇️
agent-for-php-8.3	77.38% <100.00%> (-0.56%)	⬇️
agent-for-php-8.4	77.40% <100.00%> (?)

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