[prpc] dump the original request in base64
[prpc] dump the original request in base64

Change-Id: I9e41a0ef9422ba2d1029e03c95f73449e2fcff43
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/3843051
Reviewed-by: Yuanjun Huang <a5f4b8445ce5e958d19c0a7caeadcd07c9d18770@google.com>
Commit-Queue: Chan Li <332b6c56634d0f63001046c664a2370006a02420@chromium.org>

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