fix(spanner): moved mTLSContext configurator from builder to construtor (#3605)

* fix(spanner): moved mTLSContext configurator from builder to construtor

* builder configurator properties retained when not null

Author: sagnghos

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

@@ -741,7 +741,20 @@ protected SpannerOptions(Builder builder) {
 
     transportChannelExecutorThreadNameFormat = builder.transportChannelExecutorThreadNameFormat;
     channelProvider = builder.channelProvider;
-    channelConfigurator = builder.channelConfigurator;
+    if (builder.mTLSContext != null) {
+      channelConfigurator =
+          channelBuilder -> {
+            if (builder.channelConfigurator != null) {
+              channelBuilder = builder.channelConfigurator.apply(channelBuilder);
+            }
+            if (channelBuilder instanceof NettyChannelBuilder) {
+              ((NettyChannelBuilder) channelBuilder).sslContext(builder.mTLSContext);
+            }
+            return channelBuilder;
+          };
+    } else {
+      channelConfigurator = builder.channelConfigurator;
+    }
     interceptorProvider = builder.interceptorProvider;
     sessionPoolOptions =
         builder.sessionPoolOptions != null
@@ -1620,15 +1633,6 @@ public SpannerOptions build() {
         // As we are using plain text, we should never send any credentials.
         this.setCredentials(NoCredentials.getInstance());
       }
-      if (mTLSContext != null) {
-        this.setChannelConfigurator(
-            builder -> {
-              if (builder instanceof NettyChannelBuilder) {
-                ((NettyChannelBuilder) builder).sslContext(mTLSContext);
-              }
-              return builder;
-            });
-      }
       if (this.numChannels == null) {
         this.numChannels =
             this.grpcGcpExtensionEnabled ? GRPC_GCP_ENABLED_DEFAULT_CHANNELS : DEFAULT_CHANNELS;