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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