docs: add Healthcheck API endpoints to 'REST API' page

[hassankhan]

Check List

• Tests has been run in packages where changes made if available
• Linter has been run for changed code
• Tests for the changes have been added if not covered yet
• Docs have been added / updated if required

Description of Changes Made (if issue reference is not provided)

Followup for #1607

[keydunov]

Looks good! Should we share in more details what is tested both on readyz and livez? E.g. database connection. What do you think? @hassankhan @paveltiunov @ovr

[hassankhan]

@keydunov That sounds like a great idea 👍

WDYT of the following?:

/readyz

Returns the ready state of the deployment. Ensures that a successful connection is set up between Cube.js and its' cache and queue.

/livez

Returns the liveness state of the deployment. Ensures that a successful connection is set up between Cube.js and all of the configured data sources.

[keydunov]

There are some details about the testing database connection with single-tenant setup vs multi-tenant. AFAIK readyz does the database test as well, but only for single-tenant applications.

[ovr]

Should we add details that:

For single tenant: It's open a new OrchestratorApi (I don't know how to name it in documentation) for default datasource to test connection?
For multi tenancy: It checks only created connections per tenant via OrchestratorApi (datasources), if no connections exist it will return OK.

How it stores connections:

SingleTenant:
    - OrchestratorApi (multi sources can exists)
       - DataSource1
       - DataSource2

MultiTenant:
    - Tenant1
        - OrchestratorApi (multi sources can exists)
           - DataSource1
           - DataSource2
     - Tenant2
        - OrchestratorApi (multi sources can exists)
           - DataSource1
           - DataSource2

P.S

It's test connection to queue/cache driver, but it's similar for now.