ref(tracing): keep old span name as op instead of default

[lcian]

Description

I am not sure the changes in #887 are fully beneficial, as it might have been useful to users to see the module where each span comes from at a glance in the UI.
For that reason, this PR changes the default op to be "<span target>::<span name>" which becomes "<module>::<function>" for spans created via tracing::instrument.

Combining the changes in this and the previous PR, the effect is that, for tracing::instrument spans, the name and op will be simply swapped with respect to the previous release.

Result in the UI:

Previous version for comparison:

#skip-changelog as I'm updating the previous changelog entry

[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 73.45%. Comparing base (75aff83) to head (2198c50).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #905      +/-   ##
==========================================
- Coverage   73.46%   73.45%   -0.01%     
==========================================
  Files          64       64              
  Lines        7517     7518       +1     
==========================================
  Hits         5522     5522              
- Misses       1995     1996       +1     

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