[cipd] Log parameters of SearchInstances RPC.
[cipd] Log parameters of SearchInstances RPC.

Will be used to find out what clients will need to migrate to using metadata
instead of tags.

Make logging entries have somewhat regular format, so it is easy to parse them
from BigQuery.

TBR=nodir@chromium.org

Change-Id: I8bc4615dae8b3135d4d8823d747508be72737e76
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/1639024
Reviewed-by: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org>
Commit-Queue: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go