/*!
@page bigtable-samples-grpc-credentials Example: Use Cloud Bigtable C++ client with various gRPC credential classes

# Use gRPC Credential Classes with Cloud Bigtable C++ client

## Before you begin

### Set the Environment Variable

There are many ways to authenticate with Google Cloud Platform, here we use
the `GOOGLE_APPLICATION_CREDENTIALS` environment variable to setup
authentication based on a key file:

`export GOOGLE_APPLICATION_CREDENTIALS="[PATH]"`

where [PATH] is file path for JSON file that contains either user account or Service account key. Please refer [this link][info-google-authentication] for more details on Google Authentication.

## Use Access Token Credentials

Acquire Access Token

Use [gcloud][print-access-token] to acquire an Access Token

The following code shows how to use access token to connect to an `InstanceAdmin` endpoint.

@snippet bigtable_grpc_credentials.cc test access token

## Use Refresh Token Credentials

Sample code to connect to InstanceAdmin endpoint

```
#include "google/cloud/bigtable/instance_admin.h"
#include "google/cloud/bigtable/instance_admin_client.h"

int main(int argc, char* argv[]) try {
  std::string const project_id = argv[1];
  grpc::string refresh_token = R"""({
      "client_id": "XXXXXX.apps.googleusercontent.com",
      "client_secret": "<actual secret here>",
      "refresh_token": "<actual token value here>",
      "type": "authorized_user"
  })""";
  auto call_credentials = grpc::GoogleRefreshTokenCredentials(refresh_token);
  auto channel_credentials = grpc::SslCredentials(grpc::SslCredentialsOptions());
  auto credentials = grpc::CompositeChannelCredentials(channel_credentials, call_credentials);
  auto instance_admin_client(
      google::cloud::bigtable::CreateDefaultInstanceAdminClient(
          project_id, google::cloud::bigtable::ClientOptions(credentials)));
  google::cloud::bigtable::InstanceAdmin instance_admin(instance_admin_client);
  auto instances = instance_admin.ListInstances();
  return 0;
} catch (std::exception const& ex) {
  std::cerr << "Standard C++ exception raised: " << ex.what() << std::endl;
  return 1;
}
```

## Use JWT Access Token Credentials

The following code shows how to use a JWT access token to connect to an `InstanceAdmin` endpoint.

@snippet bigtable_grpc_credentials.cc test jwt access token

## Use Google Compute Engine Credentials

The following code shows how to use a GCE credentials to connect to an `InstanceAdmin` endpoint.

@snippet bigtable_grpc_credentials.cc test gce credentials

One may face "Request had insufficient authentication scopes." error while running above example. This might be due to disabled "Cloud API access scope" for Bigtable. This error can be removed by providing sufficient access as explained [here][api-access-scope].

## Use of IAM Credentials

These credentials are not supported yet.

[info-google-authentication]: https://cloud.google.com/docs/authentication/getting-started
[info-root-certificates]: https://github.com/GoogleCloudPlatform/google-cloud-cpp/tree/master/google/cloud/bigtable/examples#configure-grpc-root-certificates
[print-access-token]: https://cloud.google.com/sdk/gcloud/reference/beta/auth/application-default/print-access-token
[api-access-scope]: https://cloud.google.com/compute/docs/access/create-enable-service-accounts-for-instances#changeserviceaccountandscopes

*/