chore(spanner): Release Multiplexed session for Partitioned Ops.

Author: pratickchokhani

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

@@ -370,9 +370,7 @@ private static Boolean getUseMultiplexedSessionFromEnvVariable() {
   @VisibleForTesting
   @InternalApi
   protected static Boolean getUseMultiplexedSessionFromEnvVariablePartitionedOps() {
-    // Checks the value of env, GOOGLE_CLOUD_SPANNER_MULTIPLEXED_SESSIONS_PARTITIONED_OPS
-    // This returns null until Partitioned Operations is supported.
-    return null;
+    return parseBooleanEnvVariable("GOOGLE_CLOUD_SPANNER_MULTIPLEXED_SESSIONS_PARTITIONED_OPS");
   }
 
   private static Boolean parseBooleanEnvVariable(String variableName) {