This allows requesting information about the server uptime and start time. This is the first patch in a series of monitoring changes, hence it's not immediately useful. Next step is propagating the index freshness information and then probably loading metadata into the index server.
The way to test new behaviour through command line:
```
$ grpc_cli call localhost:50051 Monitor/MonitoringInfo ''
connecting to localhost:50051
uptime_seconds: 42
index_age_seconds: 609568
Rpc succeeded with OK status
```
Reviewed By: kadircet
Differential Revision: https://reviews.llvm.org/D98246
This allows it to have a separate namespace (grpc versioned service) without
putting versioning info on all of the other protos (before we need it).
clang-index-server is still broken (from 81e5f298c4).
Differential Revision: https://reviews.llvm.org/D90031
Kirill Bobyrev