Guard core-metric telemetry drain against queue-full delta loss

getValueAndReset() resets a counter's delta baseline, so calling it before the
offer() succeeds means a full metricsQueue silently drops that delta forever --
worst under telemetry backpressure when the agent is unreachable. In both the
span-metrics and baggage collection loops, check remainingCapacity() before
reading each counter and stop 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

@@ -35,6 +35,12 @@ public void prepareMetrics() {
     for (SpanMetricsImpl spanMetrics : this.spanMetricRegistry.getSpanMetrics()) {
       String tag = INTEGRATION_NAME_TAG + spanMetrics.getInstrumentationName();
       for (CoreCounter counter : spanMetrics.getCounters()) {
+        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
@@ -51,6 +57,12 @@ public void prepareMetrics() {
 
     // Collect baggage metrics
     for (BaggageMetrics.TaggedCounter counter : this.baggageMetrics.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