sdk/log/observ: guard LogProcessed with Enabled

[NesterovYehor]

Guards SLP.LogProcessed with processed.Enabled(ctx) to avoid building metric
options/attributes when the processed counter is disabled.

BenchmarkSLP (noop MeterProvider):

LogProcessedWithError: ~106 ns/op, 232 B/op, 3 allocs/op -> ~0.46 ns/op, 0 B/op, 0 allocs/op
No behavior change when enabled; existing tests cover enabled path.
I don't believe this is a "notable" change, so doesn't need a CHANGELOG entry.
Issue: #7800

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.7%. Comparing base (4401cf2) to head (615c0e1).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #7848   +/-   ##
=====================================
  Coverage   81.7%   81.7%           
=====================================
  Files        304     304           
  Lines      23235   23236    +1     
=====================================
+ Hits       18993   18995    +2     
+ Misses      3860    3858    -2     
- Partials     382     383    +1     

Files with missing lines	Coverage Δ
sdk/log/internal/observ/simple_log_processor.go	100.0% <100.0%> (ø)

... and 3 files with indirect coverage changes

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