gddo-server: add logging for start and end of each request

In order to better understand what endpoints are being used
on godoc.org, an entry is now logged for the start and end of each
request. The latency of each request is also logged.

Change-Id: I8198b1206d04564e86b782e3e94964e829d5cc9b
Reviewed-on: https://go-review.googlesource.com/c/gddo/+/209223
Reviewed-by: Tuo Shan <shantuo@google.com>
2 files changed