[api] Optimise `TraceContextPropagator.Extract`

[stevejgordon]

Contributes to #728

Changes

This PR removes almost all overhead of extracting the tracestate inside the TraceContextPropagator by switching to a stack-allocated or rented buffer when building the final string. Specifically, this avoids using StringBuilder and HashSet to prevent heap allocations. The execution time is also slightly improved with this approach. I've preserved the existing behaviour based on the tests.

This also adds a benchmark to measure the performance of the extract method.

| Method     | LongListMember | MembersCount | Mean        | Error     | StdDev    | Ratio | RatioSD | Gen0    | Gen1   | Allocated | Alloc Ratio |
|----------- |--------------- |------------- |------------:|----------:|----------:|------:|--------:|--------:|-------:|----------:|------------:|
| Extract    | False          | 0            |    128.2 ns |   2.02 ns |   1.79 ns |  1.00 |    0.00 |  0.0312 |      - |     392 B |        1.00 |
| ExtractNew | False          | 0            |    122.5 ns |   2.20 ns |   2.62 ns |  0.96 |    0.02 |  0.0176 |      - |     224 B |        0.57 |
|            |                |              |             |           |           |       |         |         |        |           |             |
| Extract    | False          | 4            |    551.5 ns |  10.68 ns |  14.61 ns |  1.00 |    0.00 |  0.2108 | 0.0010 |    2656 B |        1.00 |
| ExtractNew | False          | 4            |    309.9 ns |   3.43 ns |   2.86 ns |  0.56 |    0.02 |  0.0463 |      - |     584 B |        0.22 |
|            |                |              |             |           |           |       |         |         |        |           |             |
| Extract    | False          | 32           |  3,419.8 ns | 145.07 ns | 420.86 ns |  1.00 |    0.00 |  1.2512 | 0.0381 |   15704 B |        1.00 |
| ExtractNew | False          | 32           |  2,841.3 ns |  54.61 ns |  76.56 ns |  0.83 |    0.05 |  0.2327 |      - |    2936 B |        0.19 |
|            |                |              |             |           |           |       |         |         |        |           |             |
| Extract    | True           | 0            |    129.5 ns |   2.47 ns |   3.29 ns |  1.00 |    0.00 |  0.0312 |      - |     392 B |        1.00 |
| ExtractNew | True           | 0            |    131.5 ns |   2.13 ns |   1.99 ns |  1.02 |    0.03 |  0.0176 |      - |     224 B |        0.57 |
|            |                |              |             |           |           |       |         |         |        |           |             |
| Extract    | True           | 4            |  3,052.5 ns |  60.07 ns |  59.00 ns |  1.00 |    0.00 |  1.5640 | 0.0648 |   19648 B |        1.00 |
| ExtractNew | True           | 4            |  1,972.3 ns |  38.22 ns |  35.75 ns |  0.65 |    0.03 |  0.3471 | 0.0038 |    4360 B |        0.22 |
|            |                |              |             |           |           |       |         |         |        |           |             |
| Extract    | True           | 32           | 20,789.8 ns | 409.46 ns | 438.11 ns |  1.00 |    0.00 | 10.6812 | 2.1057 |  134392 B |        1.00 |
| ExtractNew | True           | 32           | 15,939.3 ns | 313.31 ns | 505.94 ns |  0.76 |    0.03 |  2.6245 | 0.3204 |   33147 B |        0.25 |

Merge requirement checklist

• [x] CONTRIBUTING guidelines followed (license requirements, nullable enabled, static analysis, etc.)
• [x] Unit tests added/updated
• [x] Appropriate CHANGELOG.md files updated for non-trivial changes
• ~[ ] Changes in public API reviewed (if applicable)~

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 94.73684% with 3 lines in your changes missing coverage. Please review.

Project coverage is 86.22%. Comparing base (6250307) to head (d43e26f). Report is 288 commits behind head on main.

Files	Patch %	Lines
....Api/Context/Propagation/TraceContextPropagator.cs	94.73%	3 Missing :warning:

Additional details and impacted files

[](https://app.codecov.io/gh/open-telemetry/opentelemetry-dotnet/pull/5749?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=open-telemetry) ```diff @@ Coverage Diff @@ ## main #5749 +/- ## ========================================== + Coverage 83.38% 86.22% +2.84% ========================================== Files 297 256 -41 Lines 12531 11140 -1391 ========================================== - Hits 10449 9606 -843 + Misses 2082 1534 -548 ``` | [Flag](https://app.codecov.io/gh/open-telemetry/opentelemetry-dotnet/pull/5749/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=open-telemetry) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/open-telemetry/opentelemetry-dotnet/pull/5749/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=open-telemetry) | `?` | | | [unittests-Project-Experimental](https://app.codecov.io/gh/open-telemetry/opentelemetry-dotnet/pull/5749/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=open-telemetry) | `86.17% <94.73%> (?)` | | | [unittests-Project-Stable](https://app.codecov.io/gh/open-telemetry/opentelemetry-dotnet/pull/5749/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=open-telemetry) | `86.25% <94.73%> (?)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=open-telemetry#carryforward-flags-in-the-pull-request-comment) to find out more. | [Files](https://app.codecov.io/gh/open-telemetry/opentelemetry-dotnet/pull/5749?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=open-telemetry) | Coverage Δ | | |---|---|---| | [....Api/Context/Propagation/TraceContextPropagator.cs](https://app.codecov.io/gh/open-telemetry/opentelemetry-dotnet/pull/5749?src=pr&el=tree&filepath=src%2FOpenTelemetry.Api%2FContext%2FPropagation%2FTraceContextPropagator.cs&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=open-telemetry#diff-c3JjL09wZW5UZWxlbWV0cnkuQXBpL0NvbnRleHQvUHJvcGFnYXRpb24vVHJhY2VDb250ZXh0UHJvcGFnYXRvci5jcw==) | `90.82% <94.73%> (+1.35%)` | :arrow_up: | ... and [201 files with indirect coverage changes](https://app.codecov.io/gh/open-telemetry/opentelemetry-dotnet/pull/5749/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=open-telemetry)

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[stevejgordon]

@CodeBlanch, I am just bumping here to keep this open and see if we can solve the discussion on using stackalloc offline to move this along? I had hoped to attend the SIG to raise it, but the timing didn't work out again last night.

[CodeBlanch]

Sorry for the delay on this @stevejgordon! We did look at this on the SIG yesterday. @vishweshbankwar will do a review pass when he has a moment and then we'll go from there.