feat: Add #[must_use] to AsyncInstrumentBuilder

[ongardie-atomix]

This prevents code like the following from compiling:

meter
    .i64_observable_up_down_counter("my_counter")
    .with_callback(|observer| {
        observer.observe(1, &[]);
    });

This code is missing the .build() at the end. Before this commit, it would compile, but the callback would never be called. After this commit, it will cause a mostly helpful compile error:

warning: unused `AsyncInstrumentBuilder` that must be used
  --> examples/metrics-basic/src/main.rs:71:5
   |
71 | /     meter
72 | |         .i64_observable_up_down_counter("my_counter")
73 | |         .with_callback(|observer| {
74 | |             observer.observe(1, &[]);
75 | |         });
   | |__________^
   |
   = note: AsyncInstrumentBuilder must be built to receive callbacks.
   = note: `#[warn(unused_must_use)]` on by default
help: use `let _ = ...` to ignore the resulting value
   |
71 |     let _ = meter
   |     +++++++

Most builders need to be built to be useful. Normal metrics are built and then observations are reported to them, where a failure to call build() would be discovered at compile-time. However, observable metrics (async instruments) don't have that natural use point, as the result of building an AsyncInstrumentBuilder isn't typically needed. This makes AsyncInstrumentBuilder especially error-prone without the #[must_use] declaration.

Fixes #
Design discussion issue (if applicable) #

Changes

Please provide a brief description of the changes here.

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

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.8%. Comparing base (95af815) to head (720ca4a).

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #3246   +/-   ##
=====================================
  Coverage   80.8%   80.8%           
=====================================
  Files        129     129           
  Lines      23199   23199           
=====================================
+ Hits       18745   18750    +5     
+ Misses      4454    4449    -5     

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[lalitb]

Even though this breaks only wrong code, it still changes user-visible behavior. Better to add a CHANGELOG.

[Copilot]

Pull Request Overview

This PR adds the #[must_use] attribute to AsyncInstrumentBuilder to prevent silent failures when developers forget to call .build() on observable metric builders. This is particularly important for async instruments because their results are typically not stored after building (unlike sync instruments), making it easy to accidentally omit the .build() call, which would cause callbacks to never be invoked.

• Added #[must_use] attribute with helpful message to AsyncInstrumentBuilder struct

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