fix: use a new managed channel builder when creating channels (#13684)

ChannelPoolDpImpl calls the channel supplier many times over a pool's
lifetime — at warm-up, on load-driven resize, on capacity misses in
newStream, and on channel recycles.

gRPC builders are mutating: intercept(...) appends to an internal list,
and build() snapshots the current list into the new channel. So if
anything installs an interceptor on the shared builder between builds,
that interceptor accumulates. If the interceptor includes any headers,
this causes all the headers to accumulate.

Author: mutianf

java-bigtable/google-cloud-bigtable/src/main/java/com/google/cloud/bigtable/data/v2/internal/channels/SwitchingChannelPool.java

@@ -29,7 +29,6 @@
 import io.grpc.CallOptions;
 import io.grpc.ClientInterceptor;
 import io.grpc.ManagedChannel;
-import io.grpc.ManagedChannelBuilder;
 import io.grpc.MethodDescriptor;
 import java.time.Duration;
 import java.util.concurrent.ScheduledExecutorService;
@@ -190,28 +189,30 @@ private ChannelPool newChannelPoolFromProvider(
   private ChannelPool newChannelPoolFromProvider(
       ChannelProvider channelProvider, String logName, ClientInterceptor... interceptors) {
     if (channelProvider.isSingleEndpoint()) {
-      return new SingleChannelPool(
-          channelBuilderToSupplier(channelProvider.newChannelBuilder(), interceptors));
+      return new SingleChannelPool(channelSupplier(channelProvider, interceptors));
     }
 
     if (logName != null) {
       return new ChannelPoolDpImpl(
-          channelBuilderToSupplier(channelProvider.newChannelBuilder(), interceptors),
+          channelSupplier(channelProvider, interceptors),
           currentConfiguration,
           logName,
           metrics.getDebugTagTracer(),
           backgroundExecutor);
     }
 
     return new ChannelPoolDpImpl(
-        channelBuilderToSupplier(channelProvider.newChannelBuilder(), interceptors),
+        channelSupplier(channelProvider, interceptors),
         currentConfiguration,
         metrics.getDebugTagTracer(),
         backgroundExecutor);
   }
 
-  private Supplier<ManagedChannel> channelBuilderToSupplier(
-      ManagedChannelBuilder<?> channelBuilder, ClientInterceptor... interceptors) {
-    return () -> channelBuilder.intercept(interceptors).build();
+  // Each supplier invocation must produce a fresh ManagedChannelBuilder. Capturing one builder
+  // and calling .build() on it repeatedly lets anything that mutates the builder between builds
+  // accumulate — the Nth channel then ships with N copies of that interceptor.
+  private Supplier<ManagedChannel> channelSupplier(
+      ChannelProvider channelProvider, ClientInterceptor... interceptors) {
+    return () -> channelProvider.newChannelBuilder().intercept(interceptors).build();
   }
 }