[clangd] Add basic monitoring info request for remote index server

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