Log Watchdog events to scuba (#85391)
Summary: This diff logs some events of FileTimerServer to a scuba table. The events include "server started", "server stopped", "set timer", "clear timer" and "kill worker process".
Test Plan:
### Unit Test
```
buck test mode/dev-nosan //caffe2/test/distributed/elastic/agent/server/test:local_agent_test
```
```
Test Session: https://www.internalfb.com/intern/testinfra/testrun/1407375146936031
RE: reSessionID-2224cf79-6a28-4762-ab7c-9875adb244dc 3.4 KiB▲, 0.0 B▼
Jobs completed: 57. Time elapsed: 3084.4s.
Tests finished: Pass 55. Fail 0. Fatal 0. Skip 0. 0 builds failed
```
Differential Revision: D39665560
Pull Request resolved: https://github.com/pytorch/pytorch/pull/85391
Approved by: https://github.com/d4l3k
