This commit adds a new `StartupStep` interface and its factory
`ApplicationStartup`. Such steps are created, tagged with metadata and
thir execution time can be recorded - in order to collect metrics about
the application startup.
The default implementation is a "no-op" variant and has no side-effect.
Other implementations can record and collect events in a dedicated
metrics system or profiling tools. We provide here an implementation for
recording and storing steps with Java Flight Recorder.
This commit also instruments the Spring application context to gather
metrics about various phases of the application context, such as:
* context refresh phase
* bean definition registry post-processing
* bean factory post-processing
* beans instantiation and post-processing
Third part libraries involved in the Spring application context can
reuse the same infrastructure to record similar metrics.
Closes gh-24878
9301d7a294