CRAS: server_metrics - Fix a bug in sending message

The length in the message header should be length of the message.

BUG=None
TEST=use cras_test_client --playback <file> and press p to cause
     long delay. After the stream is removed, check debug message
     that handler has handle metric message.

Signed-off-by: Cheng-Yi Chiang <cychiang@chromium.org>
Change-Id: I8167358a39c9abaf5b62e246fb4c76cc76ecb37f
Reviewed-on: https://chromium-review.googlesource.com/411742
Reviewed-by: Dylan Reid <dgreid@chromium.org>
3 files changed