metric: don't drain timer that just fired
metric: don't drain timer that just fired

I mistakenly thought of the Timer as Ticker so after the first timeout
it would hang for 1h before it could flush metrics again.

Thanks Dave for helping me debug this.

Tested on prod.

Fixes #136

Change-Id: Id791ba8507859867fd209ad68edc493fee410a10
Reviewed-on: https://upspin-review.googlesource.com/7280
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@google.com>

upspin/upspin,upspin/upspin