Update monitoring

Sentry has changed to use `sentry_sdk` instead of `raven`.https://github.com/getsentry/sentry-pythonLet's update:

- [x] https://github.com/lablup/backend.ai-error-monitor
Also let's update and refine statistics monitoring with Datadog as well.For example,

- The current number of asyncio Tasks per worker process
- The current number of connections in the database connection pool
- The number of events for each HTTP status codes in the API responses
- etc.
Let's update and apply:

- [ ] https://github.com/lablup/backend.ai-stats-monitor
We also need to apply Sentry/Datadog to our Javascript-side components:

- [ ] https://github.com/lablup/backend.ai-console
- [ ] https://github.com/lablup/backend.ai-wsproxy
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JIRA Issue: BA-325

Start by reviewing the linked backend.ai-error-monitor, backend.ai-stats-monitor, backend.ai-console, and backend.ai-wsproxy repositories, along with the Sentry Python reference. Determine which components already use monitoring and which need Sentry or Datadog coverage; done means the listed repositories are updated with the requested monitoring and statistics.

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