Collect bounded stats collapse counters before the unbounded span registry

The span-metric registry holds one entry per instrumentation name and can
exceed the 1024-entry telemetry queue on its own. Draining it before the small,
fixed set of client-side stats collapse counters could fill the queue and
starve those counters indefinitely. Collect them first so they are always
emitted; the pre-read capacity guard still protects their delta baseline.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: dougqh

internal-api/src/main/java/datadog/trace/api/telemetry/CoreMetricCollector.java

@@ -33,6 +33,32 @@ private CoreMetricCollector() {
 
   @Override
   public void prepareMetrics() {
+    // Collect the bounded, high-value client-side trace-stats span-collapse counters first, tagged
+    // by collapse reason. There is only a small, fixed set of these; the span-metric registry below
+    // is unbounded (one entry per instrumentation name), so draining it first could fill the queue
+    // and starve the collapse counters indefinitely under high instrumentation counts. Collecting
+    // them up front guarantees they are emitted.
+    for (StatsMetrics.TaggedCounter counter : this.statsMetrics.getTaggedCounters()) {
+      if (this.metricsQueue.remainingCapacity() == 0) {
+        // Queue full: stop before reading any more counters. getValueAndReset() below resets the
+        // counter's delta baseline, so resetting one we then fail to enqueue would drop that delta
+        // for good; the untouched counters are picked up on the next collection cycle.
+        break;
+      }
+      long value = counter.getValueAndReset();
+      if (value == 0) {
+        // Skip not updated counters
+        continue;
+      }
+      CoreMetric metric =
+          new CoreMetric(
+              METRIC_NAMESPACE, true, counter.getName(), "count", value, counter.getTag());
+      if (!this.metricsQueue.offer(metric)) {
+        // Stop adding metrics if the queue is full
+        break;
+      }
+    }
+
     // Collect span metrics
     for (SpanMetricsImpl spanMetrics : this.spanMetricRegistry.getSpanMetrics()) {
       String tag = INTEGRATION_NAME_TAG + spanMetrics.getInstrumentationName();
@@ -67,28 +93,6 @@ public void prepareMetrics() {
         break;
       }
     }
-
-    // Collect client-side trace-stats span-collapse metrics, tagged by collapse reason.
-    for (StatsMetrics.TaggedCounter counter : this.statsMetrics.getTaggedCounters()) {
-      if (this.metricsQueue.remainingCapacity() == 0) {
-        // Queue full: stop before reading any more counters. getValueAndReset() below resets the
-        // counter's delta baseline, so resetting one we then fail to enqueue would drop that
-        // delta for good; the untouched counters are picked up on the next collection cycle.
-        break;
-      }
-      long value = counter.getValueAndReset();
-      if (value == 0) {
-        // Skip not updated counters
-        continue;
-      }
-      CoreMetric metric =
-          new CoreMetric(
-              METRIC_NAMESPACE, true, counter.getName(), "count", value, counter.getTag());
-      if (!this.metricsQueue.offer(metric)) {
-        // Stop adding metrics if the queue is full
-        break;
-      }
-    }
   }
 
   @Override