[k8s] The configuration file for the custom nginx.conf example in GKE (with gRPC) is invalid.

The current [document](https://cloud.google.com/endpoints/docs/grpc/custom-nginx) provides a nginx.conf file that does not support gRPC. (And endpoints-samples/k8s/ does not contains such `.conf`)

`nginx.conf` that supports gRPC should look something like this (auto-generated by start_esp):

```conf
# Auto-generated by start_esp
# Copyright 2017 Google Inc.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
# http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.

daemon off;

user nginx nginx;

pid /var/run/nginx.pid;

# Worker/connection processing limits
worker_processes 1;
worker_rlimit_nofile 10240;
events { worker_connections 10240; }

# Logging to stderr enables better integration with Docker and GKE/Kubernetes.
error_log stderr warn;

http {
include /etc/nginx/mime.types;
server_tokens off;
client_max_body_size 32m;
client_body_buffer_size 128k;

# HTTP subrequests
endpoints_resolver 8.8.8.8;
endpoints_certificates /etc/nginx/trusted-ca-certificates.crt;

set_real_ip_from 0.0.0.0/0;
set_real_ip_from 0::/0;
real_ip_header X-Forwarded-For;
real_ip_recursive on;

server {
server_name "";

listen 9000 http2 backlog=16384;

access_log /dev/stdout;

location / {
# Begin Endpoints v2 Support
endpoints {
on;
server_config /etc/nginx/server_config.pb.txt;
metadata_server http://169.254.169.254;
}
# End Endpoints v2 Support

# WARNING: only first backend is used
grpc_pass 127.0.0.1:8000 override;
}

include /var/lib/nginx/extra/*.conf;
}

server {
# expose /nginx_status and /endpoints_status but on a different port to
# avoid external visibility / conflicts with the app.
listen 8090;
location /nginx_status {
stub_status on;
access_log off;
}
location /endpoints_status {
endpoints_status;
access_log off;
}
location /healthz {
return 200;
access_log off;
}
location / {
root /dev/null;
}
}
}
```

So I suggest adding the above `.conf` file.

Start by checking endpoints-samples/k8s/ and the custom-nginx documentation linked in the issue to confirm where the missing configuration belongs. Add the proposed gRPC-capable nginx.conf example, and consider the work done when the sample configuration is present and supports the documented GKE gRPC setup.

Written by the indexing model from the issue text.