feat: export SDK crate version as constant

[the-wondersmith]

Changes

PR adds a simple constant for the SDK crate's version (pulled directly from the crate's manifest [ref]).

This is specifically useful in conjunction with things like populating the value of telemetry.sdk.version when building a Resource (for example).

Merge requirement checklist

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

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: the-wondersmith / name: Mark S. (ec8a5c2, 255010c, a1757fc)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.9%. Comparing base (68af3bb) to head (ec8a5c2).

Additional details and impacted files

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##            main   #2504   +/-   ##
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  Coverage   77.9%   77.9%           
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  Files        123     123           
  Lines      22944   22944           
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  Hits       17880   17880           
  Misses      5064    5064           

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[cijothomas]

we try to keep public API as lean as possible, so unless there is a strong reason to expose this, we should not.
Users rarely need this as this is automatically populated.

[the-wondersmith]

FWIW, my specific usecase involves setting the value of telemetry.sdk.version (as defined in opentelemetry_semantic_conventions), e.x.:

Specifically, that's in a library crate intended for use in other "actual" crates to simplify setting up tracing with export to an OTLP collector, so it's a layer removed even from the opentelemetry_sdk crate itself and users are likely to not even be pulling in the sdk crate themselves, so I can't really have them supply it.

Including this simple constant just makes it much easier to "stay in line" with the defined convention without having to constantly remember to update that static string whenever the SDK version changes. Plus, cargo does do a certain amount of fudging with version numbers, so unless I pin explicitly to something like opentelemetry_sdk = "= 0.27.0" the actual version of the SDK that ends up getting used could be anything in the 0.27.* range. Much easier and more reliable to just... export a simple string constant, don't you think? 😅

Oh, and not to potentially talk past the close, but this simple constant won't meaningfully inflate the "weight" of the crate's public API at all - unless it's explicitly used in some downstream crate it won't be included in compiled binaries. In terms of compile-time overhead, cargo takes the time to determine and set that value unconditionally, so it's no extra overhead there either 🙂

[cijothomas]

Thanks for your interest in contributing to the project!
I've requested changes. Please see https://github.com/open-telemetry/opentelemetry-rust/pull/2504/files#r1913817079