[AB-353] reducing chance of concurrent modification exception when registering a new metric on the fly

abstracto-application/core/core-impl/src/main/java/dev/sheldan/abstracto/core/metric/JDAMetrics.java

@@ -33,9 +33,9 @@ public class JDAMetrics extends ListenerAdapter {
                             .tagList(Arrays.asList(MetricTag.getTag(EVENT_CLASS, eventName)))
                             .name(JDA_EVENT_METRIC)
                             .build();
-            log.info("Registering new metric for event {}. There are now {} metrics.", eventName, coveredEvents.size());
             metricService.registerCounter(metric, "Events of type " + eventName);
             coveredEvents.put(event.getClass(), metric);
+            log.info("Registering new metric for event {}. There are now {} metrics.", eventName, coveredEvents.size());
         }
         metricService.incrementCounter(coveredEvents.get(event.getClass()));
     }

abstracto-application/core/metrics-impl/src/main/java/dev/sheldan/abstracto/core/metric/service/PrometheusMetricServiceBean.java

@@ -6,6 +6,7 @@ import org.springframework.beans.factory.annotation.Autowired;
 import org.springframework.stereotype.Component;
 
 import java.util.ArrayList;
+import java.util.Collections;
 import java.util.List;
 import java.util.Optional;
 import java.util.function.Supplier;
@@ -15,14 +16,15 @@ import java.util.function.ToDoubleFunction;
 @Slf4j
 public class PrometheusMetricServiceBean implements MetricService {
 
-    private final List<Counter> counters = new ArrayList<>();
+    private final List<Counter> counters = Collections.synchronizedList(new ArrayList<>());
-    private final List<Gauge> gauges = new ArrayList<>();
+    private final List<Gauge> gauges = Collections.synchronizedList(new ArrayList<>());
 
     @Autowired
     private MeterRegistry registry;
 
     @Override
     public void registerCounter(CounterMetric metric, String description) {
+        synchronized (counters) {
             if(doesCounterExist(metric)) {
                 throw new IllegalArgumentException("Counter metric already exists.");
             }
@@ -36,6 +38,7 @@ public class PrometheusMetricServiceBean implements MetricService {
                     .register(registry);
             counters.add(counter);
         }
+    }
 
     @Override
     public void registerGauge(CounterMetric counterMetric, Supplier<Number> f, String help) {
@@ -44,11 +47,11 @@ public class PrometheusMetricServiceBean implements MetricService {
 
     @Override
     public void registerGauge(CounterMetric counterMetric, Supplier<Number> f, String help, String baseUnit) {
+        synchronized (gauges) {
             List<Tag> micrometerTags = new ArrayList<>();
             counterMetric.getTagList().forEach(metricTag ->
                     micrometerTags.add(Tag.of(metricTag.getKey(), metricTag.getValue()))
             );
-
             Gauge gauge = Gauge
                     .builder(counterMetric.getName(), f)
                     .tags(micrometerTags)
@@ -58,6 +61,8 @@ public class PrometheusMetricServiceBean implements MetricService {
             gauges.add(gauge);
         }
 
+    }
+
     @Override
     public <T> void registerGauge(CounterMetric counterMetric, T obj, ToDoubleFunction<T> f, String help) {
         registerGauge(counterMetric, obj, f, help, null);
@@ -65,11 +70,11 @@ public class PrometheusMetricServiceBean implements MetricService {
 
     @Override
     public <T> void registerGauge(CounterMetric counterMetric, T obj, ToDoubleFunction<T> f, String help, String baseUnit) {
+        synchronized (gauges) {
             List<Tag> micrometerTags = new ArrayList<>();
             counterMetric.getTagList().forEach(metricTag ->
                     micrometerTags.add(Tag.of(metricTag.getKey(), metricTag.getValue()))
             );
-
             Gauge gauge = Gauge
                     .builder(counterMetric.getName(), obj, f)
                     .tags(micrometerTags)
@@ -78,6 +83,7 @@ public class PrometheusMetricServiceBean implements MetricService {
                     .register(registry);
             gauges.add(gauge);
         }
+    }
 
     @Override
     public void incrementCounter(CounterMetric counterMetric) {
@@ -86,12 +92,14 @@ public class PrometheusMetricServiceBean implements MetricService {
 
     @Override
     public void incrementCounter(CounterMetric counterMetric, Long amount) {
+        synchronized (counters) {
             Optional<Counter> counterOptional = counters.stream().filter(counter -> compareCounterIdAndCounterMetric(counter.getId(), counterMetric)).findFirst();
             counterOptional.ifPresent(counter -> counter.increment(amount));
             if(!counterOptional.isPresent()) {
                 log.warn("Trying to increment counter {} with tags {}, which was not available (yet).", counterMetric.getName(), String.join(counterMetric.getTagList().toString()));
             }
         }
+    }
 
     private boolean doesCounterExist(CounterMetric counterMetric) {
         return counters.stream().anyMatch(counter -> compareCounterIdAndCounterMetric(counter.getId(), counterMetric));