chore: disable tracking where sessions are checked out (GoogleCloudPlatform#3156)

Skip tracking where sessions are checked out of the pool, as there should
be no need to debug session leaks in PGAdapter. This feature creates an
exception for every sesion that is checked out, which wastes a few CPU
cycles for every transaction.

Author: olavloite

src/main/java/com/google/cloud/spanner/PGAdapterSessionPoolOptionsHelper.java

@@ -27,6 +27,9 @@ private PGAdapterSessionPoolOptionsHelper() {}
   @InternalApi
   public static SessionPoolOptions.Builder useMultiplexedSessions(
       SessionPoolOptions.Builder builder) {
+    // Disable tracking where regular sessions are checked out, as debugging session leaks in
+    // PGAdapter should not be necessary.
+    builder.setTrackStackTraceOfSessionCheckout(false);
     return builder.setUseMultiplexedSession(true);
   }
 }