PT-2052 - Tools should default to unbuffered stdout/stderr

- Added option --[no]buffer-stdout, enabled by default

bin/pt-diskstats

@@ -601,6 +601,10 @@ sub get_opts {
       }
    }
 
+   if ( exists $self->{opts}->{'buffer-stdout'} && $self->{opts}->{'buffer-stdout'}->{got} ) {
+      STDOUT->autoflush(1 - $self->{opts}->{'buffer-stdout'}->{value});
+   }
+
    if ( @ARGV && $self->{strict} ) {
       $self->save_error("Unrecognized command-line options @ARGV");
    }
@@ -5474,6 +5478,13 @@ L<"SYNOPSIS"> and usage information for details.
 
 =over
 
+=item --[no]buffer-stdout
+
+default: yes
+
+Enables STDOUT buffering. Disable this option if you want to see live progress
+when using output post-processing tools such as C<tee> or C<kubectl logs>.
+
 =item --columns-regex
 
 type: string; default: .