proposal: get endpoint and credentials from builder

Ask the SpannerClientBuilder the endpoint and credentials that will be
used based on the given EmulatorDetection argument.

apis/Google.Cloud.Spanner.Data/Google.Cloud.Spanner.Data/SpannerClientCreationOptions.cs

@@ -76,7 +76,7 @@ private static async Task<ChannelCredentials> CreatedScopedDefaultCredentials()
 
         internal SpannerClientCreationOptions(SpannerConnectionStringBuilder builder)
         {
-            Endpoint = builder.EndPoint;
+            Endpoint = SpannerClientBuilder.MaybeEmulatorEndpoint(builder.EmulatorDetection) ?? builder.EndPoint;
             _credentialsFile = builder.CredentialFile;
             _credentialsOverride = builder.CredentialOverride;
             EmulatorDetection = builder.EmulatorDetection;
@@ -135,6 +135,11 @@ public override string ToString()
         /// </summary>
         internal async Task<ChannelCredentials> GetCredentialsAsync()
         {
+            var emulatorCredentials = SpannerClientBuilder.MaybeEmulatorCredentials(EmulatorDetection);
+            if (emulatorCredentials != null)
+            {
+                return emulatorCredentials;
+            }
             if (_credentialsOverride != null)
             {
                 return _credentialsOverride;

apis/Google.Cloud.Spanner.V1/Google.Cloud.Spanner.V1/SpannerClientBuilder.cs

@@ -13,6 +13,8 @@
 // limitations under the License.
 
 using Google.Api.Gax;
+using Grpc.Core;
+using System;
 using System.Threading;
 using System.Threading.Tasks;
 
@@ -34,6 +36,35 @@ public new EmulatorDetection EmulatorDetection
             set => base.EmulatorDetection = value;
         }
 
+        /// <summary>
+        /// Returns the endpoint that would be used under certain emulator detection states.
+        /// </summary>
+        /// <remarks>
+        /// If <see cref="EmulatorDetection.None"/> or <see cref="EmulatorDetection.ProductionOnly"/>,
+        /// returns null. Otherwise, returns the value of the emulator host environment variable.
+        /// </remarks>
+        public static string MaybeEmulatorEndpoint(EmulatorDetection emulatorDetection)
+        {
+            if (emulatorDetection is EmulatorDetection.None) { return null; }
+            if (emulatorDetection is EmulatorDetection.ProductionOnly) { return null; }
+            return Environment.GetEnvironmentVariable(s_emulatorHostEnvironmentVariable);
+        }
+
+        /// <summary>
+        /// Returns the credentials that would be used under certain emulator detection states.
+        /// </summary>
+        /// <remarks>
+        /// If <see cref="EmulatorDetection.None"/> or <see cref="EmulatorDetection.ProductionOnly"/>,
+        /// returns null. Otherwise, returns the credentials that would be used to connect to the
+        /// emulator.
+        /// </remarks>
+        public static ChannelCredentials MaybeEmulatorCredentials(EmulatorDetection emulatorDetection)
+        {
+            if (emulatorDetection is EmulatorDetection.None) { return null; }
+            if (emulatorDetection is EmulatorDetection.ProductionOnly) { return null; }
+            return Grpc.Core.ChannelCredentials.Insecure;
+        }
+
         private const string s_emulatorHostEnvironmentVariable = "SPANNER_EMULATOR_HOST";
         private static readonly string[] s_emulatorEnvironmentVariables = { s_emulatorHostEnvironmentVariable };