Add warning about assigning the uninstaller to '_' (open-telemetry#372)

It is common rust convention to use _ to signify an unused variable. Because we _are_ using this (until the end of the block) it is helpful to explain the intent.

Author: arlyon

README.md

@@ -37,6 +37,9 @@ use opentelemetry::{exporter::trace::stdout, trace::Tracer};
 
 fn main() -> Result<(), Box<dyn std::error::Error + Send + Sync + 'static>> {
     // Create a new instrumentation pipeline
+    // Note: uninstalling the tracer happens when the _uninstall
+    // variable is dropped. Assigning it to _ will immediately
+    // drop it and uninstall the tracer
     let (tracer, _uninstall) = stdout::new_pipeline().install();
 
     tracer.in_span("doing_work", |cx| {

opentelemetry-contrib/src/trace/exporter/datadog/mod.rs

@@ -285,5 +285,6 @@ impl trace::SpanExporter for DatadogExporter {
 }
 
 /// Uninstalls the Datadog pipeline on drop
+#[must_use]
 #[derive(Debug)]
 pub struct Uninstall(global::TracerProviderGuard);

opentelemetry-jaeger/src/lib.rs

@@ -226,6 +226,7 @@ pub fn new_pipeline() -> PipelineBuilder {
 }
 
 /// Guard that uninstalls the Jaeger trace pipeline when dropped
+#[must_use]
 #[derive(Debug)]
 pub struct Uninstall(global::TracerProviderGuard);
 

opentelemetry-otlp/src/lib.rs

@@ -232,6 +232,7 @@ impl OtlpPipelineBuilder {
 }
 
 /// Uninstalls the OTLP pipeline on drop
+#[must_use]
 #[derive(Debug)]
 pub struct Uninstall(global::TracerProviderGuard);
 

opentelemetry-zipkin/src/lib.rs

@@ -324,6 +324,7 @@ impl trace::SpanExporter for Exporter {
 }
 
 /// Uninstalls the Zipkin pipeline on drop.
+#[must_use]
 #[derive(Debug)]
 pub struct Uninstall(global::TracerProviderGuard);
 

opentelemetry/src/exporter/trace/stdout.rs

@@ -147,7 +147,8 @@ where
     }
 }
 
-/// Uninstalls the stdout pipeline on dropping.
+/// Uninstalls the stdout pipeline on drop.
+#[must_use]
 #[derive(Debug)]
 pub struct Uninstall(global::TracerProviderGuard);