[buildbucket][monitor] set DSConnectionPoolSize with 4
[buildbucket][monitor] set DSConnectionPoolSize with 4

Change-Id: I6c1e65e28b13ab56120c152ff5460f25b5f9f9f0
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/3367029
Reviewed-by: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org>
Commit-Queue: Scott Lee <1dcb292530f82960316cb3d87cd67c3f212b79cb@chromium.org>

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