perf: Optimize cloning of Context since it is immutable

[bantonsson]

Changes

This PR optimizes Context cloning and attach. It tries to mitigate the performance impact of #2821 on those operations.

Merge requirement checklist

• CONTRIBUTING guidelines followed
• Unit tests added/updated (if applicable)
• Appropriate CHANGELOG.md files updated for non-trivial, user-facing changes
• Changes in public API reviewed (if applicable)

[codecov]

Codecov Report

Attention: Patch coverage is 95.83333% with 3 lines in your changes missing coverage. Please review.

Project coverage is 81.1%. Comparing base (99cb67d) to head (9a9e5f6).

Files with missing lines	Patch %	Lines
opentelemetry/src/context.rs	95.3%	3 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #2861   +/-   ##
=====================================
  Coverage   81.1%   81.1%           
=====================================
  Files        124     124           
  Lines      23927   23961   +34     
=====================================
+ Hits       19410   19441   +31     
- Misses      4517    4520    +3     

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[scottgerring]

👷 build bot output from this run:

name	baseDuration	changesDuration	difference
context_attach/nested_cx/empty_cx	31.7±0.14ns	19.1±0.27ns	-40
context_attach/nested_cx/single_value_cx	33.7±0.22ns	19.4±0.44ns	-42
context_attach/nested_cx/span_cx	33.0±0.29ns	19.4±0.30ns	-41
context_attach/out_of_order_cx_drop/empty_cx	38.7±0.18ns	19.8±0.19ns	-49
context_attach/out_of_order_cx_drop/single_value_cx	40.1±0.13ns	20.8±0.48ns	-48
context_attach/out_of_order_cx_drop/span_cx	39.7±0.22ns	20.8±0.32ns	-48
context_attach/single_cx/empty_cx	18.1±0.22ns	12.5±0.31ns	-31
context_attach/single_cx/single_value_cx	17.6±0.08ns	12.5±0.13ns	-29
context_attach/single_cx/span_cx	17.3±0.13ns	12.5±0.25ns	-28

[bantonsson]

These are the benchmark numbers from this run:

And yes, the suppression check is slower, but on the other hand context attach and cloning is way faster.

name	baseDuration	changesDuration	difference
context/has_active_span/in-cx/alt	8.4±0.01ns	8.4±0.02ns	0.0
context/has_active_span/in-cx/spec	4.4±0.06ns	4.4±0.07ns	0.0
context/has_active_span/no-cx/alt	8.4±0.02ns	8.4±0.01ns	0.0
context/has_active_span/no-cx/spec	4.4±0.09ns	4.4±0.05ns	0.0
context/has_active_span/no-sdk/alt	8.4±0.03ns	8.4±0.01ns	0.0
context/has_active_span/no-sdk/spec	4.4±0.10ns	4.4±0.06ns	0.0
context/is_recording/in-cx/alt	4.7±0.06ns	4.7±0.06ns	0.0
context/is_recording/in-cx/spec	6.6±0.08ns	6.6±0.07ns	0.0
context/is_recording/no-cx/alt	4.7±0.05ns	4.7±0.04ns	0.0
context/is_recording/no-cx/spec	6.5±0.06ns	6.6±0.15ns	0.0
context/is_recording/no-sdk/alt	4.7±0.07ns	4.7±0.07ns	0.0
context/is_recording/no-sdk/spec	6.5±0.07ns	6.5±0.09ns	0.0
context/is_sampled/in-cx/alt	8.7±0.02ns	8.7±0.03ns	0.0
context/is_sampled/in-cx/spec	4.6±0.06ns	4.6±0.07ns	0.0
context/is_sampled/no-cx/alt	8.7±0.04ns	8.7±0.02ns	0.0
context/is_sampled/no-cx/spec	4.7±0.06ns	4.7±0.09ns	0.0
context/is_sampled/no-sdk/alt	8.7±0.03ns	8.7±0.03ns	0.0
context/is_sampled/no-sdk/spec	4.7±0.05ns	4.7±0.07ns	0.0
context_attach/nested_cx/empty_cx	44.6±0.31ns	19.2±0.80ns	-57
context_attach/nested_cx/single_value_cx	44.6±0.30ns	19.6±0.42ns	-56
context_attach/nested_cx/span_cx	44.6±0.44ns	19.5±0.20ns	-56
context_attach/out_of_order_cx_drop/empty_cx	38.5±0.40ns	20.0±0.41ns	-48
context_attach/out_of_order_cx_drop/single_value_cx	38.6±0.43ns	21.2±0.42ns	-45
context_attach/out_of_order_cx_drop/span_cx	38.6±0.45ns	21.1±0.41ns	-45
context_attach/single_cx/empty_cx	22.8±0.20ns	10.3±0.19ns	-55
context_attach/single_cx/single_value_cx	22.0±0.22ns	11.0±0.20ns	-50
context_attach/single_cx/span_cx	22.0±0.25ns	11.0±0.11ns	-50
telemetry_suppression/enter_telemetry_suppressed_scope	28.4±0.09ns	24.5±0.96ns	-14
telemetry_suppression/is_current_telemetry_suppressed_false	1.3±0.02ns	1.9±0.03ns	+47
telemetry_suppression/is_current_telemetry_suppressed_true	1.3±0.01ns	2.1±0.04ns	+68
telemetry_suppression/normal_attach	28.5±0.16ns	10.0±0.18ns	-65

[bantonsson]

This is still a draft until #2870 has been merged and all benchmarks are run properly.