[Feature] Start HTTP service when actually ready to accept connnections

[stockholmux]

Is your feature request related to a problem? Please describe.

When starting up OpenSearch Dashboards you often get the message "OpenSearch Dashboards server is not ready yet" and a 503 error code. In many environments you can detect when a port (i.e. 5601) is active but then I have to add another layer of polling to detect the 503 error which makes the transition for startup really clunky.

Additionally, it's a bad user experience to get this message.

Describe the solution you'd like

The OpenSearch Dashboards' HTTP port becomes active when it is actually able to accept connections.

Describe alternatives you've considered

Instead of the 503 error and one liner, provide loading UI element and some JS to automatically get to the login screen.

[kavilla]

Some research will have to be done, right now when it runs it's migrating documents to the current version even if they see a loading element they might be able to hit some OpenSearch Dashboards APIs.

[Kiran-Dexter]

OpenSearch Dashboards server is not ready yet

when we are starting the dashboard we are encountering this error can some shed some light here

OS = Centos 8

[ashwin-pc]

I've added context here as to why this is the case and and some possible solutions to fix it. Cross posting for better discoverability: #3181 (comment)