feat: Add description getter to metrics::Instrument in SDK

[ongardie-atomix]

Note: This change is in conflict with #2985 by @cijothomas:

Instrument currently allows users to select an instrument based on description, which is not allowed in the spec. Only name, kind, unit, scope should be exposed.

The spec at https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/metrics/sdk.md#instrument-selection-criteria says:

The SDK MAY accept additional criteria.

So it seems like the description MAY be exposed.

---

I could imagine accessing the description in a View for valid reasons, like wanting to append a "." (or remove a ".") from the end of the descriptions. I admit my motivation for doing this PR is part of a larger hack, however. I want my application to be able to list all of its metrics with descriptions, even for metrics that have been registered but not used. I think the only way I can get access to those currently is with a View, but I know that's not its intended use.

---

Changes

• Adds a getter for the description field of metrics::Instrument in the SDK.
• Updates docs based on the field definitions, which seem more descriptive, and cleans up some others.
• Does this need a CHANGELOG.md entry?

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)

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: ongardie-atomix / name: Diego Ongaro (971db88, d6632c3)

[codecov]

Codecov Report

❌ Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.8%. Comparing base (95af815) to head (971db88).

Files with missing lines	Patch %	Lines
opentelemetry-sdk/src/metrics/instrument.rs	0.0%	3 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #3245   +/-   ##
=====================================
  Coverage   80.8%   80.8%           
=====================================
  Files        129     129           
  Lines      23199   23202    +3     
=====================================
+ Hits       18745   18749    +4     
+ Misses      4454    4453    -1     

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[cijothomas]

Can you elaborate the use case more, about fixing descriptions.?

[ongardie-atomix]

Can you elaborate the use case more, about fixing descriptions.?

Sure. With this PR, someone could do something like this to make sure all their descriptions end with punctuation:

let meter_provider = meter_provider_builder
    .with_view(|instrument| {
        let description = instrument.description();
        if description.is_empty() || description.ends_with(['.', '!', '?']) {
            None
        } else {
            Stream::builder()
                .with_description(format!("{description}."))
                .build()
                .ok()
        }
    })
    .build();

I think this is similar to other use cases for views.

[ongardie-atomix]

I'll elaborate about my real motivation in another issue soon. I'm just trying to understand things a little better first.