Report poller target in the worker heartbeat

The poller target is already sent on each poll, but matching consumes it inside
the scaling decision and discards it -- nothing persists it and nothing can
observe it. Reporting it on the heartbeat too lets the server read it per worker
and export it as a metric, making fleet-wide poller distribution visible without
instrumenting every SDK.

WorkerHeartbeat already carries four WorkerPollerInfo sub-messages, so this is
per-task-type for free, including the normal-vs-sticky workflow split.

PollScaleReportHandle is a local inside AsyncPoller.start() and is not reachable
from the heartbeat builder. PollerTracker already exists to bridge exactly that
gap -- its javadoc says so -- so the target supplier goes there, and
buildPollerInfo reads it alongside the in-flight count it already reads.

AsyncNexusPollTask now overrides setPollerTargetSupplier purely to feed its
tracker. It had taken the interface's no-op default because the nexus poll
request has no field for a target, but it does have an autoscaler and a
heartbeat slot.

Fixed-size pools run MultiThreadedPoller, which has no scale report handle, so
no supplier is installed and the tracker reports 0 -- which the proto defines as
unknown. The api submodule moves to the commit carrying the new field.

Author: veeral-patel

temporal-sdk/src/main/java/io/temporal/internal/worker/AsyncActivityPollTask.java

@@ -97,6 +97,8 @@ public AsyncActivityPollTask(
   @Override
   public void setPollerTargetSupplier(IntSupplier pollerTargetSupplier) {
     this.pollerTargetSupplier = pollerTargetSupplier;
+    // Also hand it to the tracker, which is what the heartbeat builder can reach.
+    pollerTracker.setPollerTargetSupplier(pollerTargetSupplier);
   }
 
   /**

temporal-sdk/src/main/java/io/temporal/internal/worker/AsyncNexusPollTask.java

@@ -21,6 +21,7 @@
 import io.temporal.worker.tuning.SlotReleaseReason;
 import java.util.Objects;
 import java.util.concurrent.CompletableFuture;
+import java.util.function.IntSupplier;
 import java.util.function.Supplier;
 import javax.annotation.Nonnull;
 import org.slf4j.Logger;
@@ -110,6 +111,16 @@ public AsyncNexusPollTask(
     this.pollRequest = pollRequest.build();
   }
 
+  /**
+   * The nexus poll request has no field for a poller target, so unlike the workflow and activity
+   * tasks this only feeds the tracker -- the heartbeat reports a target for every autoscaling pool,
+   * not only the pools poll-time fairness acts on.
+   */
+  @Override
+  public void setPollerTargetSupplier(IntSupplier pollerTargetSupplier) {
+    pollerTracker.setPollerTargetSupplier(pollerTargetSupplier);
+  }
+
   @Override
   @SuppressWarnings("deprecation")
   public CompletableFuture<NexusTask> poll(SlotPermit permit) {

temporal-sdk/src/main/java/io/temporal/internal/worker/AsyncWorkflowPollTask.java

@@ -129,6 +129,8 @@ public AsyncWorkflowPollTask(
   @Override
   public void setPollerTargetSupplier(IntSupplier pollerTargetSupplier) {
     this.pollerTargetSupplier = pollerTargetSupplier;
+    // Also hand it to the tracker, which is what the heartbeat builder can reach.
+    pollerTracker.setPollerTargetSupplier(pollerTargetSupplier);
   }
 
   /**

temporal-sdk/src/main/java/io/temporal/internal/worker/PollerTracker.java

@@ -3,6 +3,7 @@
 import java.time.Instant;
 import java.util.concurrent.atomic.AtomicInteger;
 import java.util.concurrent.atomic.AtomicReference;
+import java.util.function.IntSupplier;
 
 /**
  * Tracks in-flight poll count and last successful poll time for heartbeat reporting.
@@ -20,6 +21,7 @@
 public class PollerTracker {
   private final AtomicInteger inFlightPolls = new AtomicInteger();
   private final AtomicReference<Instant> lastSuccessfulPollTime = new AtomicReference<>();
+  private final AtomicReference<IntSupplier> pollerTargetSupplier = new AtomicReference<>();
 
   /** Increments in-flight count. Returns the new value for forwarding to the Tally gauge. */
   public int pollStarted() {
@@ -43,4 +45,22 @@ public int getInFlightPolls() {
   public Instant getLastSuccessfulPollTime() {
     return lastSuccessfulPollTime.get();
   }
+
+  /**
+   * Supplies the autoscaler's current target for this pool. Set by the poll task once {@link
+   * AsyncPoller} has installed it, which is why this is a supplier rather than a value -- the scale
+   * report handle does not exist until the poller starts.
+   */
+  public void setPollerTargetSupplier(IntSupplier supplier) {
+    pollerTargetSupplier.set(supplier);
+  }
+
+  /**
+   * The autoscaler's current target, or 0 when this pool has a fixed size and so has no target. The
+   * heartbeat proto defines 0 as unknown.
+   */
+  public int getPollerTarget() {
+    IntSupplier supplier = pollerTargetSupplier.get();
+    return supplier == null ? 0 : supplier.getAsInt();
+  }
 }

temporal-sdk/src/main/java/io/temporal/worker/Worker.java

@@ -749,6 +749,7 @@ private static WorkerPollerInfo buildPollerInfo(
       builder.setIsAutoscaling(true);
     }
     builder.setCurrentPollers(tracker.getInFlightPolls());
+    builder.setPollerTarget(tracker.getPollerTarget());
     Instant lastPoll = tracker.getLastSuccessfulPollTime();
     if (lastPoll != null) {
       builder.setLastSuccessfulPollTime(toProtoTimestamp(lastPoll));