Encapsulate SDK Tracer observability

[MrAlias]

Split from #7316

Follow guidelines and move instrumentation into its own type.

Benchmarks

Added sdk/trace/internal/observ benchmarks

goos: linux
goarch: amd64
pkg: go.opentelemetry.io/otel/sdk/trace/internal/observ
cpu: Intel(R) Core(TM) i7-8550U CPU @ 1.80GHz
                     │ enc-trace-sdk-tracer-obs.out │
                     │            sec/op            │
Tracer/SpanStarted-8                    7.436n ± 6%
Tracer/SpanLive-8                       9.987n ± 8%
Tracer/SpanEnded-8                      11.32n ± 7%
NewTracer-8                             87.64n ± 6%
geomean                                 16.48n

                     │ enc-trace-sdk-tracer-obs.out │
                     │             B/op             │
Tracer/SpanStarted-8                   0.000 ± 0%
Tracer/SpanLive-8                      0.000 ± 0%
Tracer/SpanEnded-8                     0.000 ± 0%
NewTracer-8                            0.000 ± 0%
geomean                                           ¹
¹ summaries must be >0 to compute geomean

                     │ enc-trace-sdk-tracer-obs.out │
                     │          allocs/op           │
Tracer/SpanStarted-8                   0.000 ± 0%
Tracer/SpanLive-8                      0.000 ± 0%
Tracer/SpanEnded-8                     0.000 ± 0%
NewTracer-8                            0.000 ± 0%
geomean                                           ¹
¹ summaries must be >0 to compute geomean

Existing sdk/trace benchmarks

> benchstat main.out enc-trace-sdk-tracer-obs.out
goos: linux
goarch: amd64
pkg: go.opentelemetry.io/otel/sdk/trace
cpu: Intel(R) Core(TM) i7-8550U CPU @ 1.80GHz
                                  │  main.out   │     enc-trace-sdk-tracer-obs.out     │
                                  │   sec/op    │    sec/op     vs base                │
SpanEnd/ObservabilityEnabled-8      188.5n ± 4%   131.5n ± 32%  -30.24% (p=0.000 n=10)
TraceStart/ObservabilityEnabled-8   886.9n ± 8%   663.9n ±  2%  -25.14% (p=0.000 n=10)
geomean                             408.9n        295.5n        -27.73%

                                  │  main.out  │      enc-trace-sdk-tracer-obs.out       │
                                  │    B/op    │    B/op     vs base                     │
SpanEnd/ObservabilityEnabled-8      16.00 ± 0%    0.00 ± 0%  -100.00% (p=0.000 n=10)
TraceStart/ObservabilityEnabled-8   608.0 ± 0%   576.0 ± 0%    -5.26% (p=0.000 n=10)
geomean                             98.63                    ?                       ¹ ²
¹ summaries must be >0 to compute geomean
² ratios must be >0 to compute geomean

                                  │  main.out  │      enc-trace-sdk-tracer-obs.out       │
                                  │ allocs/op  │ allocs/op   vs base                     │
SpanEnd/ObservabilityEnabled-8      1.000 ± 0%   0.000 ± 0%  -100.00% (p=0.000 n=10)
TraceStart/ObservabilityEnabled-8   5.000 ± 0%   3.000 ± 0%   -40.00% (p=0.000 n=10)
geomean                             2.236                    ?                       ¹ ²
¹ summaries must be >0 to compute geomean
² ratios must be >0 to compute geomean

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.4%. Comparing base (e4ab314) to head (2373f5d).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #7331     +/-   ##
=======================================
- Coverage   85.4%   85.4%   -0.1%     
=======================================
  Files        267     268      +1     
  Lines      24295   24297      +2     
=======================================
+ Hits       20758   20759      +1     
- Misses      3160    3161      +1     
  Partials     377     377             

Files with missing lines	Coverage Δ
sdk/trace/internal/observ/tracer.go	100.0% <100.0%> (ø)
sdk/trace/provider.go	86.6% <100.0%> (-0.8%)	⬇️
sdk/trace/span.go	84.2% <100.0%> (-0.1%)	⬇️
sdk/trace/tracer.go	100.0% <100.0%> (ø)

... and 1 file with indirect coverage changes

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[Copilot]

Pull Request Overview

This PR encapsulates SDK Tracer observability functionality by moving instrumentation logic from the main tracer type into a dedicated internal observability package. The refactoring improves maintainability by separating concerns and provides better performance through optimized attribute caching.

• Extracts observability instrumentation into a new sdk/trace/internal/observ package
• Replaces direct metric handling in tracer with a dedicated observ.Tracer type
• Updates all references to use the new observability constants and interfaces

Reviewed Changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
sdk/trace/tracer.go	Replaces inline observability fields and logic with observ.Tracer instance
sdk/trace/provider.go	Updates tracer creation to use new observ.NewTracer() constructor
sdk/trace/span.go	Simplifies span end observability calls using new interface
sdk/trace/internal/observ/tracer.go	New observability tracer implementation with optimized attribute caching
sdk/trace/internal/observ/tracer_test.go	Comprehensive tests for the new observability functionality
sdk/trace/trace_test.go	Updates test expectations to use new observability scope constants
sdk/trace/provider_test.go	Adjusts test assertions for new observability interface
.codespellignore	Adds "observ" to ignored words list

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