Add health checks for server processes

[racheldaniel]

What is this PR?

This is a:

• documentation update
• bug fix with no breaking changes
• new functionality
• a breaking change

All pull requests from community contributors should target the main branch (default).

Description & motivation

Adds liveliness tracking using celery signals and a shell script that can be used to check readiness and liveliness. This PR is in conjunction with dbt-cloud PR #7344.

The bash script:

1. Sends an http request to the /ready endpoint of dbt-server to make sure server itself is up
2. Makes sure the broker can be accessed via the command redis-cli and sending a ping
3. Verifies that a file /tmp/worker_heartbeat exists and has been touched in the last 10 seconds (this touch command is executed once the worker is ready, then again as a part of the celery worker's steps, making use of celery signals)

These steps should confirm that the server is started and running, the redis broker can be connected to, and the celery workers are live.

Checklist

• I have verified that these changes work locally on the following warehouses (Note: it's okay if you do not have access to all warehouses, this helps us understand what has been covered)
  • BigQuery
  • Postgres
  • Redshift
  • Snowflake
  • Databricks
  • Spark
• I have updated the README.md (if applicable)
• I have added tests & descriptions to my models
• I have added an entry to CHANGELOG.md

[github-actions]

Thank you for your pull request! We could not find a changelog entry for this change. For details on how to document a change, see the contributing guide.