[loggingexporter] Rename labels to attributes

[pirgeo]

Description:
While working on #5976, I realized that there are some leftover mentions of labels in the logging exporter code, and replaced them with attributes.

[codecov]

Codecov Report

Merging #5977 (7740597) into main (25192af) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #5977   +/-   ##
=======================================
  Coverage   92.24%   92.24%           
=======================================
  Files         212      212           
  Lines       13264    13264           
=======================================
  Hits        12236    12236           
  Misses        811      811           
  Partials      217      217           

Impacted Files	Coverage Δ
...er/loggingexporter/internal/otlptext/databuffer.go	99.12% <100.00%> (ø)
exporter/loggingexporter/internal/otlptext/logs.go	100.00% <100.00%> (ø)
...orter/loggingexporter/internal/otlptext/metrics.go	100.00% <100.00%> (ø)
...porter/loggingexporter/internal/otlptext/traces.go	100.00% <100.00%> (ø)

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[TylerHelmuth]

LGTM, left 1 nit

[pirgeo]

I pushed it again without the changelog commits! Thanks for taking a look 🙂