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.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.7%. Comparing base (8d3b4cb) to head (4954cde).

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #7848     +/-   ##
=======================================
- Coverage   81.7%   81.7%   -0.1%     
=======================================
  Files        304     304             
  Lines      23252   23253      +1     
=======================================
  Hits       19011   19011             
+ Misses      3860    3859      -1     
- Partials     381     383      +2     

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

... and 4 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.