test: add json output as otel assertions custom message

[fabriziosestito]

Description

Sporadically, the test_otel integration test fails in the CI (see #916).
However, this proved to be hard to reproduce locally.
For this reason, we are adding the full output as a custom message to the failing assertion, so we can debug the flakiness in a later step.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 63.11%. Comparing base (400ca81) to head (a11c721).
Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #925   +/-   ##
=======================================
  Coverage   63.11%   63.11%           
=======================================
  Files          17       17           
  Lines        1052     1052           
=======================================
  Hits          664      664           
  Misses        388      388           

Flag	Coverage Δ
integration-tests	55.56% <ø> (ø)
unit-tests	38.28% <ø> (ø)

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