chore: add a random channel hint with single use transactions while u… (#3065)

Without the hint, GAX would do a round-robin on channels. In theory, a random hint would give a better performance than round robin at a high QPS workload. Also YCSB benchmarks suggest having the hint is better for higher QPS.

Author: arpan14

google-cloud-spanner/src/main/java/com/google/cloud/spanner/AbstractReadContext.java

@@ -190,8 +190,11 @@ private SingleReadContext(Builder builder) {
       this.bound = builder.bound;
       // single use transaction have a single RPC and hence there is no need
       // of a channel hint. GAX will automatically choose a hint when used
-      // with a multiplexed session.
-      this.channelHint = getChannelHintOptions(session.getOptions(), null);
+      // with a multiplexed session to perform a round-robin channel selection. We are
+      // passing a hint here to prefer random channel selection instead of doing GAX round-robin.
+      this.channelHint =
+          getChannelHintOptions(
+              session.getOptions(), ThreadLocalRandom.current().nextLong(Long.MAX_VALUE));
     }
 
     @Override