m/p/api: use protobuf.Timestamp in LogEntry

This updates LogEntry to use google.protobuf.Timestamp.
See: issue #129.

Change-Id: I937800aa91e86690da0d06f743e720c2d474ad0a
Reviewed-on: https://review.monogon.dev/c/monogon/+/832
Tested-by: Jenkins CI
Reviewed-by: Sergiusz Bazanski <serge@monogon.tech>
diff --git a/metropolis/proto/api/BUILD.bazel b/metropolis/proto/api/BUILD.bazel
index 5bc5640..a333b96 100644
--- a/metropolis/proto/api/BUILD.bazel
+++ b/metropolis/proto/api/BUILD.bazel
@@ -15,6 +15,7 @@
         "//metropolis/proto/common:common_proto",
         "//metropolis/proto/ext:ext_proto",
         "@com_google_protobuf//:duration_proto",
+        "@com_google_protobuf//:timestamp_proto",
     ],
 )