feature: add StatsCollector for basic metrics aggregation

src/main/java/com/volta/stats/StatsCollector.java

@@ -0,0 +1,56 @@
+package com.volta.stats;
+
+import java.util.concurrent.atomic.LongAccumulator;
+import java.util.concurrent.atomic.LongAdder;
+
+public class StatsCollector {
+  private final LongAdder totalRequests = new LongAdder();
+  private final LongAdder successCount = new LongAdder();
+  private final LongAdder totalLatencyMs = new LongAdder();
+  private final LongAccumulator maxLatencyMs = new LongAccumulator(Math::max, Long.MIN_VALUE);
+  private final LongAccumulator minLatencyMs = new LongAccumulator(Math::min, Long.MAX_VALUE);
+
+  public void record(int statusCode, long latencyMs) throws IllegalArgumentException {
+    if (statusCode < 100 || statusCode > 599) {
+      throw new IllegalArgumentException("statusCode must be integer in [100, 599]");
+    }
+    if (latencyMs < 0) {
+      throw new IllegalArgumentException("latencyMs must be non-negative");
+    }
+
+    totalRequests.increment();
+    if (200 <= statusCode && statusCode < 300) {
+      successCount.increment();
+    }
+    totalLatencyMs.add(latencyMs);
+    maxLatencyMs.accumulate(latencyMs);
+    minLatencyMs.accumulate(latencyMs);
+  }
+
+  public StatsSnapshot getSnapshot() {
+    long snapshotTotalRequests = totalRequests.sum();
+    long snapshotSuccessCount = successCount.sum();
+    long snapshotTotalLatencyMs = totalLatencyMs.sum();
+
+    long snapshotMinLatencyMs = (snapshotTotalRequests == 0 ? 0 : minLatencyMs.get());
+    long snapshotMaxLatencyMs = (snapshotTotalRequests == 0 ? 0 : maxLatencyMs.get());
+    long snapshotAvgLatencyMs =
+        (snapshotTotalRequests == 0 ? 0 : snapshotTotalLatencyMs / snapshotTotalRequests);
+
+    return new StatsSnapshot(
+        snapshotTotalRequests,
+        snapshotSuccessCount,
+        snapshotTotalRequests - snapshotSuccessCount,
+        snapshotAvgLatencyMs,
+        snapshotMinLatencyMs,
+        snapshotMaxLatencyMs);
+  }
+
+  public void reset() {
+    totalRequests.reset();
+    successCount.reset();
+    totalLatencyMs.reset();
+    minLatencyMs.reset();
+    maxLatencyMs.reset();
+  }
+}

src/main/java/com/volta/stats/StatsSnapshot.java

@@ -0,0 +1,9 @@
+package com.volta.stats;
+
+public record StatsSnapshot(
+    long totalRequests,
+    long successCount,
+    long errorCount,
+    long avgLatencyMs,
+    long minLatencyMs,
+    long maxLatencyMs) {}