dotnet tools shouldn't error if no arguments are provided

[davidfowl]

Just print the help and drop the "Required command was not provided." .e.g

root@004561cfab39:/# dotnet counters
Required command was not provided.

Usage:
  dotnet-counters [options] [command]

Options:
  --version    Display version information

Commands:
  monitor <counter_list>    Start monitoring a .NET application
  list                      Display a list of counter names and descriptions, grouped by provider.
  list-processes            Display a list of dotnet processes that can be monitored.