Do not suppress 0 values

0 is a valid value for a gauge or a counter (e.g. "number of active sessions"
can easily be zero and that state is distinct from is distinct 1) and zero
samples in a period is legit for a histogram over an interval.

For gauges we should also not reset to zero on each render, but we should
continue to do so for counters.

Suppression of idle metrics should be done using `StatsdBuilder::idle_timeout`.

Fixes dialtone#5

src/builder.rs

@@ -261,10 +261,6 @@ impl StatsdBuilder {
     ///
     /// Refer to the documentation for [`MetricKindMask`](metrics_util::MetricKindMask) for more
     /// information on defining a metric kind mask.
-    ///
-    /// When a metric is rendered its value is replaced with a "zero-value" for that `MetricKind`
-    /// however any metric with a state "zero-value" will not be rendered and will be cleaned up
-    /// when its corresponding idle timeout expires.
     #[must_use]
     pub fn idle_timeout(mut self, mask: MetricKindMask, timeout: Option<Duration>) -> Self {
         self.idle_timeout = timeout;
@@ -614,7 +610,12 @@ mod tests {
         gauge1.set(-3.44);
         let rendered = handle.render();
         // each render call will reset the value of the counter
-        let expected_gauge = "basic.gauge:-3.44|g|#wutang:forever\n\n";
+        let expected_gauge = concat!(
+            "basic.counter:0|c\n",
+            "\n",
+            "basic.gauge:-3.44|g|#wutang:forever\n",
+            "\n",
+        );
         assert_eq!(rendered, expected_gauge);
 
         let key = Key::from_name("basic.histogram");
@@ -623,6 +624,8 @@ mod tests {
         let rendered = handle.render();
 
         let histogram_data = concat!(
+            "basic.counter:0|c\n\n",
+            "basic.gauge:-3.44|g|#wutang:forever\n\n",
             "basic.histogram.min:12|g\n",
             "basic.histogram.max:12|g\n",
             "basic.histogram.avg:12|g\n",

src/recorder.rs

@@ -51,7 +51,7 @@ impl Inner {
             }
 
             let (name, labels) = key_to_parts(&key, Some(&self.global_tags));
-            let value = f64::from_bits(gauge.get_inner().swap(0, Ordering::Acquire));
+            let value = f64::from_bits(gauge.get_inner().load(Ordering::Acquire));
             let entry = gauges
                 .entry(name)
                 .or_insert_with(HashMap::new)
@@ -103,9 +103,6 @@ impl Inner {
         for (name, mut by_labels) in counters.drain() {
             let mut wrote = false;
             for (labels, value) in by_labels.drain() {
-                if value == 0 {
-                    continue;
-                }
                 wrote = true;
                 write_metric_line::<&str, u64>(
                     &mut output,
@@ -128,9 +125,6 @@ impl Inner {
         for (name, mut by_labels) in gauges.drain() {
             let mut wrote = false;
             for (labels, value) in by_labels.drain() {
-                if value == 0.0 {
-                    continue;
-                }
                 wrote = true;
                 write_metric_line::<&str, f64>(
                     &mut output,
@@ -156,9 +150,6 @@ impl Inner {
                 let (sum, count) = match distribution {
                     Distribution::Summary(summary, quantiles, sum) => {
                         let count = summary.count();
-                        if count == 0 {
-                            continue;
-                        }
                         wrote = true;
                         let snapshot = summary.snapshot(Instant::now());
                         for quantile in quantiles.iter() {
@@ -192,9 +183,6 @@ impl Inner {
                     }
                     Distribution::Histogram(histogram) => {
                         let count = histogram.count();
-                        if count == 0 {
-                            continue;
-                        }
                         wrote = true;
                         for (le, count) in histogram.buckets() {
                             write_metric_line(