Allow subclasses of DropwizardExports to deal with validation errors

In the current form, if e.g. a Counter contains invalid data the entire
`collect` calls fails with InvalidArgumentException.

In large applications, it may be preferable to gracefully degrade the
available metrics and report the failure via e.g. logging. This change
allows just that - by changing the access modifiers to the `fromXYZ` methods
subclasses can now handle the errors.

Finally, to allow for skipping of invalid metrics, we follow the example
in `fromGauge` to interpret the meaning of a returned null to be that the
metric should be skipped.

Author: mnssn

prometheus-metrics-instrumentation-dropwizard/src/main/java/io/prometheus/metrics/instrumentation/dropwizard/DropwizardExports.java

@@ -85,7 +85,7 @@ private MetricMetadata getMetricMetaData(String metricName, Metric metric) {
    * Export counter as Prometheus <a
    * href="https://prometheus.io/docs/concepts/metric_types/#gauge">Gauge</a>.
    */
-  MetricSnapshot fromCounter(String dropwizardName, Counter counter) {
+  protected MetricSnapshot fromCounter(String dropwizardName, Counter counter) {
     MetricMetadata metadata = getMetricMetaData(dropwizardName, counter);
     CounterSnapshot.CounterDataPointSnapshot.Builder dataPointBuilder =
         CounterSnapshot.CounterDataPointSnapshot.builder()
@@ -99,7 +99,7 @@ MetricSnapshot fromCounter(String dropwizardName, Counter counter) {
   }
 
   /** Export gauge as a prometheus gauge. */
-  MetricSnapshot fromGauge(String dropwizardName, Gauge<?> gauge) {
+  protected MetricSnapshot fromGauge(String dropwizardName, Gauge<?> gauge) {
     Object obj = gauge.getValue();
     double value;
     if (obj instanceof Number) {
@@ -134,7 +134,7 @@ MetricSnapshot fromGauge(String dropwizardName, Gauge<?> gauge) {
    * @param count the total sample count for this snapshot.
    * @param factor a factor to apply to histogram values.
    */
-  MetricSnapshot fromSnapshotAndCount(
+  protected MetricSnapshot fromSnapshotAndCount(
       String dropwizardName, Snapshot snapshot, long count, double factor, String helpMessage) {
     Quantiles quantiles =
         Quantiles.builder()
@@ -159,7 +159,7 @@ MetricSnapshot fromSnapshotAndCount(
   }
 
   /** Convert histogram snapshot. */
-  MetricSnapshot fromHistogram(String dropwizardName, Histogram histogram) {
+  protected MetricSnapshot fromHistogram(String dropwizardName, Histogram histogram) {
     return fromSnapshotAndCount(
         dropwizardName,
         histogram.getSnapshot(),
@@ -169,7 +169,7 @@ MetricSnapshot fromHistogram(String dropwizardName, Histogram histogram) {
   }
 
   /** Export Dropwizard Timer as a histogram. Use TIME_UNIT as time unit. */
-  MetricSnapshot fromTimer(String dropwizardName, Timer timer) {
+  protected MetricSnapshot fromTimer(String dropwizardName, Timer timer) {
     return fromSnapshotAndCount(
         dropwizardName,
         timer.getSnapshot(),
@@ -179,7 +179,7 @@ MetricSnapshot fromTimer(String dropwizardName, Timer timer) {
   }
 
   /** Export a Meter as a prometheus COUNTER. */
-  MetricSnapshot fromMeter(String dropwizardName, Meter meter) {
+  protected MetricSnapshot fromMeter(String dropwizardName, Meter meter) {
     MetricMetadata metadata = getMetricMetaData(dropwizardName + "_total", meter);
     CounterSnapshot.CounterDataPointSnapshot.Builder dataPointBuilder =
         CounterSnapshot.CounterDataPointSnapshot.builder().value(meter.getCount());
@@ -207,17 +207,40 @@ public MetricSnapshots collect() {
 
     registry
         .getCounters(metricFilter)
-        .forEach((name, counter) -> metricSnapshots.metricSnapshot(fromCounter(name, counter)));
+        .forEach(
+            (name, counter) -> {
+              MetricSnapshot snapshot = fromCounter(name, counter);
+              if (snapshot != null) {
+                metricSnapshots.metricSnapshot(snapshot);
+              }
+            });
     registry
         .getHistograms(metricFilter)
         .forEach(
-            (name, histogram) -> metricSnapshots.metricSnapshot(fromHistogram(name, histogram)));
+            (name, histogram) -> {
+              MetricSnapshot snapshot = fromHistogram(name, histogram);
+              if (snapshot != null) {
+                metricSnapshots.metricSnapshot(snapshot);
+              }
+            });
     registry
         .getTimers(metricFilter)
-        .forEach((name, timer) -> metricSnapshots.metricSnapshot(fromTimer(name, timer)));
+        .forEach(
+            (name, timer) -> {
+              MetricSnapshot snapshot = fromTimer(name, timer);
+              if (snapshot != null) {
+                metricSnapshots.metricSnapshot(snapshot);
+              }
+            });
     registry
         .getMeters(metricFilter)
-        .forEach((name, meter) -> metricSnapshots.metricSnapshot(fromMeter(name, meter)));
+        .forEach(
+            (name, meter) -> {
+              MetricSnapshot snapshot = fromMeter(name, meter);
+              if (snapshot != null) {
+                metricSnapshots.metricSnapshot(snapshot);
+              }
+            });
 
     return metricSnapshots.build();
   }

prometheus-metrics-instrumentation-dropwizard/src/test/java/io/prometheus/metrics/instrumentation/dropwizard/DropwizardExportsTest.java

@@ -7,6 +7,7 @@
 import com.codahale.metrics.*;
 import io.prometheus.metrics.expositionformats.OpenMetricsTextFormatWriter;
 import io.prometheus.metrics.model.registry.PrometheusRegistry;
+import io.prometheus.metrics.model.snapshots.MetricSnapshot;
 import io.prometheus.metrics.model.snapshots.SummarySnapshot;
 import java.io.ByteArrayOutputStream;
 import java.io.IOException;
@@ -278,6 +279,52 @@ public void testThatMetricHelpUsesOriginalDropwizardName() {
     assertThat(convertToOpenMetricsFormat()).isEqualTo(expected);
   }
 
+  @Test
+  void responseWhenRegistryIsEmpty() {
+    var registry = new PrometheusRegistry();
+    registry.register(new IgnoreInvalidCounter(metricRegistry));
+    assertThat(convertToOpenMetricsFormat(registry))
+        .isEqualTo(
+            """
+# EOF
+""");
+  }
+
+  @Test
+  void collectInvalidMetricFails() {
+    metricRegistry.counter("my.application.namedCounter1").inc(-10);
+    assertThatThrownBy(() -> convertToOpenMetricsFormat())
+        .isInstanceOf(IllegalArgumentException.class);
+  }
+
+  @Test
+  void allowSubclassesToIgnoreInvalidMetric() {
+    var registry = new PrometheusRegistry();
+    registry.register(new IgnoreInvalidCounter(metricRegistry));
+    metricRegistry.counter("my.application.namedCounter1").inc(-10);
+    assertThat(convertToOpenMetricsFormat(registry))
+        .isEqualTo(
+            """
+# EOF
+""");
+  }
+
+  private static class IgnoreInvalidCounter extends DropwizardExports {
+
+    public IgnoreInvalidCounter(MetricRegistry registry) {
+      super(registry);
+    }
+
+    @Override
+    protected MetricSnapshot fromCounter(String dropwizardName, Counter counter) {
+      try {
+        return super.fromCounter(dropwizardName, counter);
+      } catch (IllegalArgumentException exc) {
+        return null;
+      }
+    }
+  }
+
   private static class ExampleDoubleGauge implements Gauge<Double> {
     @Override
     public Double getValue() {