Annotate metric types with must_use (#475)

Author: Dav1dde

metrics/src/handles.rs

@@ -40,18 +40,21 @@ pub trait HistogramFn {
 
 /// A counter.
 #[derive(Clone)]
+#[must_use = "counters do nothing unless you use them"]
 pub struct Counter {
     inner: Option<Arc<dyn CounterFn + Send + Sync>>,
 }
 
 /// A gauge.
 #[derive(Clone)]
+#[must_use = "gauges do nothing unless you use them"]
 pub struct Gauge {
     inner: Option<Arc<dyn GaugeFn + Send + Sync>>,
 }
 
 /// A histogram.
 #[derive(Clone)]
+#[must_use = "histograms do nothing unless you use them"]
 pub struct Histogram {
     inner: Option<Arc<dyn HistogramFn + Send + Sync>>,
 }

metrics/tests/macros/02_trailing_comma.rs

@@ -12,9 +12,9 @@ fn no_trailing_comma() {
 
 #[allow(dead_code)]
 fn with_trailing_comma() {
-    counter!("qwe",);
+    counter!("qwe",).increment(1);
     counter!(
-        "qwe", 
+        "qwe",
         "foo" => "bar",
     ).increment(1);
     counter!("qwe", vec![],).increment(1);

metrics/tests/macros/03_mod_aliasing.rs

@@ -13,7 +13,7 @@ use framework::*; // This exposes mod `framework::metrics`.
 
 #[inline]
 pub fn register_metrics() {
-    ::metrics::counter!(
+    let _ = ::metrics::counter!(
         metrics::UPLOAD_METRIC_NAME,
         &[
             (metrics::UPLOAD_METRIC_LABEL_PROCESS_TYPE, ""),