chore: Expose more details for use in client libraries

The GcpCallInvoker constructor signature is fairly longwinded, but we only use it in a few places, so it's unlikely that it will need to change.

Google.Api.Gax.Grpc/ClientBuilderBase.cs

@@ -462,7 +462,12 @@ internal async Task<GoogleCredential> GetGoogleCredentialAsync(CancellationToken
         /// </summary>
         protected abstract ChannelPool GetChannelPool();
 
-        private GrpcAdapter EffectiveGrpcAdapter => GrpcAdapter ?? GrpcAdapter.GetFallbackAdapter(ServiceMetadata);
+        /// <summary>
+        /// Returns the effective <see cref="GrpcAdapter"/> for this builder,
+        /// using the <see cref="GrpcAdapter"/> property if that is set, or the appropriate fallback adapter
+        /// for <see cref="ServiceMetadata"/> otherwise.
+        /// </summary>
+        protected GrpcAdapter EffectiveGrpcAdapter => GrpcAdapter ?? GrpcAdapter.GetFallbackAdapter(ServiceMetadata);
 
         /// <summary>
         /// Returns the options to use when creating a channel, taking <see cref="GrpcChannelOptions"/>

Google.Api.Gax.Grpc/Gcp/GcpCallInvoker.cs

@@ -20,7 +20,7 @@ namespace Google.Api.Gax.Grpc.Gcp
     /// Call invoker which can fan calls out to multiple underlying channels
     /// based on request properties.
     /// </summary>
-    internal sealed class GcpCallInvoker : CallInvoker
+    public sealed class GcpCallInvoker : CallInvoker
     {
         private static int s_clientChannelIdCounter;
 
@@ -41,15 +41,15 @@ internal sealed class GcpCallInvoker : CallInvoker
         private readonly ServiceMetadata _serviceMetadata;
 
         /// <summary>
-        /// Initializes a new instance of the <see cref="Grpc.Gcp.GcpCallInvoker"/> class.
+        /// Initializes a new instance.
         /// </summary>
         /// <param name="serviceMetadata">The metadata for the service that this call invoker will be used with. Must not be null.</param>
         /// <param name="target">Target of the underlying grpc channels. Must not be null.</param>
         /// <param name="credentials">Credentials to secure the underlying grpc channels. Must not be null.</param>
         /// <param name="options">Channel options to be used by the underlying grpc channels. Must not be null.</param>
         /// <param name="apiConfig">The API config to apply. Must not be null.</param>
         /// <param name="adapter">The adapter to use to create channels. Must not be null.</param>
-        internal GcpCallInvoker(ServiceMetadata serviceMetadata, string target, ChannelCredentials credentials, GrpcChannelOptions options, ApiConfig apiConfig, GrpcAdapter adapter)
+        public GcpCallInvoker(ServiceMetadata serviceMetadata, string target, ChannelCredentials credentials, GrpcChannelOptions options, ApiConfig apiConfig, GrpcAdapter adapter)
         {
             _serviceMetadata = GaxPreconditions.CheckNotNull(serviceMetadata, nameof(serviceMetadata));
             _target = GaxPreconditions.CheckNotNull(target, nameof(target));

Google.Api.Gax.Grpc/Gcp/GcpCallInvokerPool.cs

@@ -67,7 +67,7 @@ public Task ShutdownChannelsAsync()
         /// <param name="apiConfig">The API configuration used to determine channel keys. Must not be null.</param>
         /// <param name="adapter">The gRPC adapter to use to create call invokers. Must not be null.</param>
         /// <returns>A call invoker for the specified endpoint.</returns>
-        public CallInvoker GetCallInvoker(string endpoint, GrpcChannelOptions options, ApiConfig apiConfig, GrpcAdapter adapter)
+        public GcpCallInvoker GetCallInvoker(string endpoint, GrpcChannelOptions options, ApiConfig apiConfig, GrpcAdapter adapter)
         {
             GaxPreconditions.CheckNotNull(endpoint, nameof(endpoint));
             var credentials = _credentialsCache.GetCredentials();
@@ -86,7 +86,7 @@ public CallInvoker GetCallInvoker(string endpoint, GrpcChannelOptions options, A
         /// <param name="adapter">The gRPC adapter to use to create call invokers. Must not be null.</param>
         /// <returns>A task representing the asynchronous operation. The value of the completed
         /// task will be a call invoker for the specified endpoint.</returns>
-        public async Task<CallInvoker> GetCallInvokerAsync(string endpoint, GrpcChannelOptions options, ApiConfig apiConfig, GrpcAdapter adapter)
+        public async Task<GcpCallInvoker> GetCallInvokerAsync(string endpoint, GrpcChannelOptions options, ApiConfig apiConfig, GrpcAdapter adapter)
         {
             GaxPreconditions.CheckNotNull(endpoint, nameof(endpoint));
             GaxPreconditions.CheckNotNull(apiConfig, nameof(apiConfig));

Google.Api.Gax.Grpc/GrpcAdapter.cs

@@ -38,9 +38,8 @@ private protected GrpcAdapter(ApiTransports supportedTransports)
             _supportedTransports = supportedTransports;
         }
 
-        // TODO: potentially expose either or both of the methods below if we want, or a
-        // "get the first adapter that supports this service, from the following list, or a fallback"
-        // method.
+        // TODO: potentially expose this if we want, or a "get the first adapter that supports
+        // this service, from the following list, or a fallback" method.
 
         /// <summary>
         /// Returns whether or not this adapter supports the specified service.
@@ -58,7 +57,7 @@ internal bool SupportsApi(ServiceMetadata serviceMetadata)
         /// </summary>
         /// <param name="serviceMetadata">The descriptor of the API. Must not be null.</param>
         /// <returns>A suitable GrpcAdapter for the given API, preferring the use of the binary gRPC transport where available.</returns>
-        internal static GrpcAdapter GetFallbackAdapter(ServiceMetadata serviceMetadata) =>
+        public static GrpcAdapter GetFallbackAdapter(ServiceMetadata serviceMetadata) =>
             // TODO: Do we need some way of indicating a preference, if the service supports both Rest and Grpc?
             // Probably not: the user code can always specify the adapter in the builder
             serviceMetadata.Transports.HasFlag(ApiTransports.Grpc) ? s_defaultGrpcTransportFactory.Value