Add startup Actuator endpoint

[bclozel]

As of #22603, developers can configure a BufferingApplicationStartup on their SpringApplication and collect application startup events.

While this feature can be used by a custom component to push this data in an external metrics system - we should provide a dedicated Actuator endpoint that drains the buffered data and exports it as a JSON document for further analysis.

[wilkinsona]

It's unusual for a @ReadOperation (which maps to a GET request with HTTP access) to have side-effects. I wonder if it should be a @WriteOperation (POST request when exposed over HTTP) of some form? The goal of that would be to make the draining behaviour clearer and also align a bit better with conventional HTTP semantics.

[bclozel]

Good call, I'll change that to a POST. I don't see the point of adding a fetch-only variant (i.e. not draining) but at least this change leaves this option on the table.

[qian0205]

hello, I want to use the startup endpoint of actuator, but i don't know how to configure and code. Can you post the code for reference, Thanks!

[bclozel]

@qian0205 this section of the reference documentation should help. Thanks!