chore: abstract plain text settings into a SpannerOption

Author: sagnghos

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

@@ -825,7 +825,7 @@ protected SpannerOptions(Builder builder) {
     openTelemetry = builder.openTelemetry;
     enableApiTracing = builder.enableApiTracing;
     enableExtendedTracing = builder.enableExtendedTracing;
-    if (builder.isExperimentalHost) {
+    if (builder.experimentalHost != null) {
       enableBuiltInMetrics = false;
     } else {
       enableBuiltInMetrics = builder.enableBuiltInMetrics;
@@ -1047,7 +1047,8 @@ public static class Builder
     private boolean enableBuiltInMetrics = SpannerOptions.environment.isEnableBuiltInMetrics();
     private String monitoringHost = SpannerOptions.environment.getMonitoringHost();
     private SslContext mTLSContext = null;
-    private boolean isExperimentalHost = false;
+    private String experimentalHost = null;
+    private boolean usePlainText = false;
     private TransactionOptions defaultTransactionOptions = TransactionOptions.getDefaultInstance();
 
     private static String createCustomClientLibToken(String token) {
@@ -1550,10 +1551,13 @@ public Builder setHost(String host) {
 
     @ExperimentalApi("https://github.com/googleapis/java-spanner/pull/3676")
     public Builder setExperimentalHost(String host) {
+      if (usePlainText && !host.startsWith("http")) {
+        host = "http://" + host;
+      }
       super.setHost(host);
       super.setProjectId(EXPERIMENTAL_HOST_PROJECT_ID);
       setSessionPoolOption(SessionPoolOptions.newBuilder().setExperimentalHost().build());
-      this.isExperimentalHost = true;
+      experimentalHost = host;
       return this;
     }
 
@@ -1614,6 +1618,22 @@ public Builder useClientCert(String clientCertificate, String clientCertificateK
       return this;
     }
 
+    /**
+     * {@code usePlainText} is only supported for experimental spanner hosts. This will configure
+     * the transport to use plaintext (no TLS) and will set credentials to {@link
+     * com.google.cloud.NoCredentials} to avoid sending authentication over an unsecured channel.
+     */
+    @ExperimentalApi("https://github.com/googleapis/java-spanner/pull/3574")
+    public Builder usePlainText() {
+      this.setChannelConfigurator(ManagedChannelBuilder::usePlaintext)
+          .setCredentials(NoCredentials.getInstance());
+      if (this.experimentalHost != null && !this.experimentalHost.startsWith("http")) {
+        this.experimentalHost = "http://" + this.experimentalHost;
+        super.setHost(this.experimentalHost);
+      }
+      return this;
+    }
+
     /**
      * Sets OpenTelemetry object to be used for Spanner Metrics and Traces. GlobalOpenTelemetry will
      * be used as fallback if this options is not set.
@@ -1789,7 +1809,7 @@ public SpannerOptions build() {
         this.setChannelConfigurator(ManagedChannelBuilder::usePlaintext);
         // As we are using plain text, we should never send any credentials.
         this.setCredentials(NoCredentials.getInstance());
-      } else if (isExperimentalHost && credentials == null) {
+      } else if (experimentalHost != null && credentials == null) {
         credentials = environment.getDefaultExperimentalHostCredentials();
       }
       if (this.numChannels == null) {

google-cloud-spanner/src/main/java/com/google/cloud/spanner/testing/ExperimentalHostHelper.java

@@ -16,10 +16,8 @@
 
 package com.google.cloud.spanner.testing;
 
-import com.google.cloud.NoCredentials;
 import com.google.cloud.spanner.SpannerOptions;
 import com.google.common.base.Strings;
-import io.grpc.ManagedChannelBuilder;
 
 public class ExperimentalHostHelper {
   private static final String EXPERIMENTAL_HOST = "spanner.experimental_host";
@@ -58,9 +56,7 @@ public static void setExperimentalHostSpannerOptions(SpannerOptions.Builder buil
     builder.setExperimentalHost(experimentalHost);
     builder.setBuiltInMetricsEnabled(false);
     if (usePlainText) {
-      builder
-          .setChannelConfigurator(ManagedChannelBuilder::usePlaintext)
-          .setCredentials(NoCredentials.getInstance());
+      builder.usePlainText();
     }
     if (isMtlsSetup()) {
       String clientCertificate = System.getProperty(CLIENT_CERT_PATH, "");