Guard stats telemetry drain against delta loss when the queue is full

getValueAndReset() resets a counter's delta baseline, so calling it before the
offer() succeeds means a full metricsQueue silently drops that delta forever --
exactly when it matters most (telemetry backing up because the agent is
unreachable). Check remainingCapacity() before reading any counter and stop the
drain early instead; the untouched counters are collected on the next cycle.

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

Author: dougqh

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

@@ -70,6 +70,12 @@ public void prepareMetrics() {
 
     // 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