sdk/trace/internal/observ: guard SpanStarted and spanLive with Enabled

[kouji-yoshimura]

Guards SpanStarted and spanLive with Enabled(ctx) to avoid building metric
options/attributes when instruments are disabled.

BenchmarkTracer (noop MeterProvider):

  SpanStarted: ~1.77 ns/op -> ~0.48 ns/op (~3.7x)
  SpanLive:    ~2.05 ns/op -> ~0.52 ns/op (~3.9x)
  SpanEnded:   ~2.05 ns/op -> ~0.52 ns/op (~3.9x)

No behavior change when enabled; existing tests cover enabled path.
No CHANGELOG entry, following #7848.

Issue: #7800

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: kouji-yoshimura / name: yoshimura (752aa1e)

[kouji-yoshimura]

@cla-bot check

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.6%. Comparing base (972a6e5) to head (752aa1e).
⚠️ Report is 7 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #8067   +/-   ##
=====================================
  Coverage   81.6%   81.6%           
=====================================
  Files        304     304           
  Lines      23446   23452    +6     
=====================================
+ Hits       19135   19140    +5     
- Misses      3929    3930    +1     
  Partials     382     382           

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

... and 5 files with indirect coverage changes

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