system_api: dbus: rename metrics_event proto file

Naming the protobuf specification "service.proto" causes conflicts
in the chrome build.  This may be related to

https://github.com/protocolbuffers/protobuf/issues/2520

namespace protobuf_service_2eproto {
void InitDefaults() {

Naming the file "service.proto" makes the namespace contain the word
"service", which happens to be used by another protobuf for vm_concierge.
So I get a link-time multiple definition.

BUG=chromium:729335
TEST=compiles and runs correctly with chromium changes that use it

Change-Id: Ic83489d3bab15f627288e44c323e5032da76d874
Reviewed-on: https://chromium-review.googlesource.com/1199965


Commit-Ready: Luigi Semenzato <semenzato@chromium.org>
Tested-by: Luigi Semenzato <semenzato@chromium.org>
Reviewed-by: Dan Erat <derat@chromium.org>