Update documentation for Metrics API (#2280)

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

Author: 3 people

opentelemetry/src/global/metrics.rs

@@ -15,6 +15,8 @@ fn global_meter_provider() -> &'static RwLock<GlobalMeterProvider> {
 
 /// Sets the given [`MeterProvider`] instance as the current global meter
 /// provider.
+///
+/// **NOTE:** This function should be called before getting [`Meter`] instances via [`meter()`] or [`meter_with_scope()`]. Otherwise, you could get no-op [`Meter`] instances.
 pub fn set_meter_provider<P>(new_provider: P)
 where
     P: metrics::MeterProvider + Send + Sync + 'static,
@@ -44,6 +46,9 @@ pub fn meter_provider() -> GlobalMeterProvider {
 /// Creates a named [`Meter`] via the currently configured global [`MeterProvider`].
 ///
 /// This is a more convenient way of expressing `global::meter_provider().meter(name)`.
+///
+/// **NOTE:** Calls to [`meter()`] return a [`Meter`] backed by the global [`MeterProvider`] configured during the method invocation.
+/// If the global [`MeterProvider`] is changed after getting [`Meter`] instances from these calls, the [`Meter`] instances returned will not reflect the change.
 pub fn meter(name: &'static str) -> Meter {
     meter_provider().meter(name)
 }
@@ -52,6 +57,9 @@ pub fn meter(name: &'static str) -> Meter {
 ///
 /// This is a simpler alternative to `global::meter_provider().meter_with_scope(...)`
 ///
+/// **NOTE:** Calls to [`meter_with_scope()`] return a [`Meter`] backed by the global [`MeterProvider`] configured during the method invocation.
+/// If the global [`MeterProvider`] is changed after getting [`Meter`] instances from these calls, the [`Meter`] instances returned will not reflect the change.
+///
 /// # Example
 ///
 /// ```