Use available port for gRPC server tests

[marcotc]

gRPC tests sometimes fail with this error:

E1219 23:49:07.801805162    2827 server_chttp2.cc:40]        {
  "description": "No address added out of total 1 resolved",
  "file": "src/core/ext/transport/chttp2/server/chttp2_server.cc:394",
  "referenced_errors": [
    {
      "description": "Address already in use",
      "errno": 98,
      "file": "src/core/lib/iomgr/tcp_server_utils_posix_common.cc:181",
      "os_error": "Address already in use",
      "syscall": "bind"
    }
  ]
}

Failures:

  1) gRPC integration test multiple client configurations uses the correct configuration information
     Failure/Error: server.add_http2_port(address, :this_port_is_insecure)
     
     RuntimeError:
       could not add port 0.0.0.0:50052 to server, not sure why
     # ./spec/ddtrace/contrib/grpc/support/grpc_helper.rb:28:in `runner'
     # ./spec/ddtrace/contrib/grpc/support/grpc_helper.rb:5:in `run_request_reply'
     # ./spec/ddtrace/contrib/grpc/integration_spec.rb:35:in `block (3 levels) in <top (required)>'

This happens when we try to bind a gRPC server to new local endpoint that is already in use.

Given we hard-coded the ports, it is possible that another process will randomly choose to bind to these high ports by chance.

The best way to bind to an available local port is to bind to port 0, letting the OS find a port for us, but GRPC::RpcServer does not support that, raising an error instead when provided with 0.

So we do the next best thing which is to find an available port ourselves and quickly hand it to gRPC. There's a very small chance that another process will bind to the port we just found available between the time we retrieve its number and hand it to gRPC, but there's no perfect solution with the APIs we are provided by the gem.

[delner]

Seems pretty straightforward! I think if we can add a comment this is good to merge.

[delner]

👍