feat: Add #[must_use] to AsyncInstrumentBuilder (#3246)

Co-authored-by: Cijo Thomas <[email protected]>
Co-authored-by: Cijo Thomas <[email protected]>

Author: ongardie-atomix

opentelemetry/CHANGELOG.md

@@ -6,6 +6,7 @@
 - **Breaking** Removed the following public fields and methods from the `SpanBuilder` [#3227][3227]:
   - `trace_id`, `span_id`, `end_time`, `status`, `sampling_result`
   - `with_trace_id`, `with_span_id`, `with_end_time`, `with_status`, `with_sampling_result`
+- **Added** `#[must_use]` attribute to `opentelemetry::metrics::AsyncInstrumentBuilder` to add compile time warning when `.build()` is not called on observable instrument builders, preventing silent failures where callbacks are never registered and metrics are never reported.
 
 [3227]: https://github.com/open-telemetry/opentelemetry-rust/pull/3227
 
@@ -117,10 +118,10 @@ let counter = meter.u64_counter("my_counter").build();
   - Replaced `global::meter_with_version` with `global::meter_with_scope`
   - Added `global::tracer_with_scope`
   - Refer to PR description for migration guide.
-- **Breaking change**: replaced `InstrumentationScope` public attributes by getters [#2275](https://github.com/open-telemetry/opentelemetry-rust/pull/2275)  
+- **Breaking change**: replaced `InstrumentationScope` public attributes by getters [#2275](https://github.com/open-telemetry/opentelemetry-rust/pull/2275)
 
 - **Breaking change**: [#2260](https://github.com/open-telemetry/opentelemetry-rust/pull/2260)
-  - Removed `global::set_error_handler` and `global::handle_error`. 
+  - Removed `global::set_error_handler` and `global::handle_error`.
   - `global::handle_error` usage inside the opentelemetry crates has been replaced with `global::otel_info`, `otel_warn`, `otel_debug` and `otel_error` macros based on the severity of the internal logs.
   - The default behavior of `global::handle_error` was to log the error using `eprintln!`. With otel macros, the internal logs get emitted via `tracing` macros of matching severity. Users now need to configure a `tracing` layer/subscriber to capture these logs.
   - Refer to PR description for migration guide. Also refer to [self-diagnostics](https://github.com/open-telemetry/opentelemetry-rust/tree/main/examples/self-diagnostics) example to learn how to view internal logs in stdout using `tracing::fmt` layer.
@@ -220,7 +221,7 @@ to learn how to provide Observable callbacks.
   opaque string. Migration: Replace `.with_unit(Unit::new("myunit"))` with
   `.with_unit("myunit")`.
 
-- [1869](https://github.com/open-telemetry/opentelemetry-rust/pull/1869) Introduced the `LogRecord::set_target()` method in the log bridge API. 
+- [1869](https://github.com/open-telemetry/opentelemetry-rust/pull/1869) Introduced the `LogRecord::set_target()` method in the log bridge API.
 This method allows appenders to set the target/component emitting the logs.
 
 ## v0.23.0
@@ -241,7 +242,7 @@ This method allows appenders to set the target/component emitting the logs.
         - opentelemetry::global::shutdown_logger_provider
         - opentelemetry::global::logger_provider
         - opentelemetry::global::GlobalLoggerProvider
-        - opentelemetry::global::ObjectSafeLoggerProvider 
+        - opentelemetry::global::ObjectSafeLoggerProvider
     For creating appenders using Logging bridge API, refer to the opentelemetry-tracing-appender [example](https://github.com/open-telemetry/opentelemetry-rust/blob/main/opentelemetry-appender-tracing/examples/basic.rs)
 
 ### Changed

opentelemetry/src/metrics/instruments/mod.rs

@@ -241,6 +241,7 @@ impl<T> fmt::Debug for HistogramBuilder<'_, T> {
 pub type Callback<T> = Box<dyn Fn(&dyn AsyncInstrument<T>) + Send + Sync>;
 
 /// Configuration for building an async instrument.
+#[must_use = "Callbacks will not be invoked unless you call .build() on this async instrument builder."]
 #[non_exhaustive] // We expect to add more configuration fields in the future
 pub struct AsyncInstrumentBuilder<'a, I, M> {
     /// Instrument provider is used to create the instrument.