chore(spanner): add environment variable for enabling read-write multiplexed sessions

Author: harshachinta

.github/workflows/ci.yaml

@@ -54,6 +54,7 @@ jobs:
         JOB_TYPE: test
         GOOGLE_CLOUD_SPANNER_MULTIPLEXED_SESSIONS: true
         GOOGLE_CLOUD_SPANNER_MULTIPLEXED_SESSIONS_PARTITIONED_OPS: true
+        GOOGLE_CLOUD_SPANNER_MULTIPLEXED_SESSIONS_FOR_RW: true
   units-java8:
     # Building using Java 17 and run the tests with Java 8 runtime
     name: "units (8)"
@@ -94,6 +95,7 @@ jobs:
           JOB_TYPE: test
           GOOGLE_CLOUD_SPANNER_MULTIPLEXED_SESSIONS: true
           GOOGLE_CLOUD_SPANNER_MULTIPLEXED_SESSIONS_PARTITIONED_OPS: true
+          GOOGLE_CLOUD_SPANNER_MULTIPLEXED_SESSIONS_FOR_RW: true
   windows:
     runs-on: windows-latest
     steps:

.github/workflows/integration-tests-against-emulator-with-multiplexed-session.yaml

@@ -40,3 +40,4 @@ jobs:
         JOB_TYPE: test
         SPANNER_EMULATOR_HOST: localhost:9010
         GOOGLE_CLOUD_SPANNER_MULTIPLEXED_SESSIONS: true
+        GOOGLE_CLOUD_SPANNER_MULTIPLEXED_SESSIONS_FOR_RW: true

.kokoro/presubmit/integration-multiplexed-sessions-enabled.cfg

@@ -41,3 +41,8 @@ env_vars: {
     key: "GOOGLE_CLOUD_SPANNER_MULTIPLEXED_SESSIONS_PARTITIONED_OPS"
     value: "true"
 }
+
+env_vars: {
+    key: "GOOGLE_CLOUD_SPANNER_MULTIPLEXED_SESSIONS_FOR_RW"
+    value: "true"
+}

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

@@ -391,9 +391,7 @@ private static Boolean parseBooleanEnvVariable(String variableName) {
   }
 
   private static Boolean getUseMultiplexedSessionForRWFromEnvVariable() {
-    // Checks the value of env, GOOGLE_CLOUD_SPANNER_MULTIPLEXED_SESSIONS_FOR_RW
-    // This returns null until RW is supported.
-    return null;
+    return parseBooleanEnvVariable("GOOGLE_CLOUD_SPANNER_MULTIPLEXED_SESSIONS_FOR_RW");
   }
 
   Duration getMultiplexedSessionMaintenanceDuration() {